DIL disappearrs to her parents all the time

No, it’s not to be condemned but it has to be agreed by both parties - one person can’t unilaterally decide they’ll stay at home while the other goes out to work full time to bring in the family income.

I think it's lovely that DIL has a good relationship with her DP.
I don't know if the OP has come back but lots of new parents still choose to SAH and look after their DC. That is their choice and life isn't necessarily about getting back to employment.

Not for everyone of course and most parents want to work as soon as possible and are looking for a good balance of affordable good childcare and a satisfying job to pay the bills but being a SAHP can be a career choice and not to be condemned.

After being married I spent as much time as I could with my parents. You really don't know someone until you live with them and he made my life a complete misery. Is there any chance that your son is bullying her?

My daughter comes to stay with the kids for weekends on a regular basis. I don't see any problem with your DIL staying with her parents. Not everyone wants to live in each other's pockets!

This is really none of your business and is not something you should be asking about or your son telling you about.

A parent's instinct is to protect a child. That's okay. That's normal. That's what parents do. But it also means that by definition, a parent is not going to be an objective third party in a child's relationship. If your son is having issues with his wife, he needs to see an objective third party (such as a licensed marriage counsellor) who can help them to figure out the issue.

To be fair though is it really any of your business. Presume you don’t know why she goes. Surely it’s between your ds and his wife, to sort out

Like Dilly I think this sounds familiar but I cannot remember the name of OP on that occasion.

Perhaps DIL is helping her family, a parent who is unwell, elderly grandparents, siblings.
MIL does not really sound like someone she might confide in really as I am sure she is seeing everything on the side of her DS.
Just one point of view.

Why is this your business, did DS ask advise?

We need some clarification from you, meet before we can begin to give any constructive advice. You haven't replied yet so we're all in the dark.

Yennifer no, I know that - but without more detail from the OP it’s impossible to know if the wife could work to help increase the family income but simply chooses not to, or whether she actually can’t for some reason.

meet you have given a bit of information but you don't really say what the problem is. Are you saying there is conflict because they are short of money? Easier to make constructive comments when the problem is made clear.

This is between your son and his wife

People can't always live in the long term sadly. This is how poor families get screwed over. Forced to buy cheap that won't last whereas if they bought more expensive they would save money long term, get a better warranty etc. A poor family might replace a 200 quid cooker 4 times in 10 years costing 800 while a 400 quid cooker lasts 15. This is not a costed analogy but still true x

This sounds like a post from last year I remember.
I think that the early years are hard and lonely, I was lucky and my mil became my children’s childcare so I could go back to work ( and also enjoy adult conversation ) perhaps she is struggling.
Also how much help is your son at home? Does he support her when he gets home? Mine didn’t, and it caused seething resentment ( he still doesn’t help with housework even though he is retired and I’m still working).
Luckily for my pregnant dil I think my elder son is a bit more modern and will be pulling his weight, but not everyone is so lucky. Don’t take what son is telling you on face value as he isn’t likely to tell you the full picture if he isn’t being supportive.

Obviously the OP hasn’t clarified if he’s offered to be the SAHP but she has confirmed that the wife doesn’t want to work. Time for them to sit down as a couple and agree roles and responsibilities that make them both happy.

Has the OPs son offered to be the SAHP?

If not, its not fair to criticise the mother for not working

I think this is something they need to sort out themselves meet unless you have been asked for your input. Your daughter in law may be planning on working when the child is older or they may not have thought that far ahead. Leave them to it would be my advice.

She IS contributing to household income by being the primary childcarer at home. She is saving them thousands

Not in the long term - by not working she’s costing them thousands lost pension revenue, loss of earning potential and so on.

Ultimately, if both adults agree to one of them not working and spending one week in three away then fine - but both have to be happy with that arrangement and there has to be ongoing discussions about how the finances are managed, when the other plans to resume work, how pension contributions for the one not working will be made and so on. It can’t and shouldn’t fall to one person (usually the man for some reason) to shoulder that without a plan or an agreement that it’s working for both of them and that they are each happy with the allocated role of wage earner/SAHP.

I've been raising children for a long time and only went back to work a few years ago. How this country is set up means that I am worse off working. While I wasn't working my husbands pay was topped up with housing benefit, counsel tax support and tax credits. (for those who might have a moan about that, families like mine cost the average wage earner 50p a a day and nothing if you earn under 20 thousand or so). Anyway I am now working which is a good example to my children and I have a pension. They have taken away what I earn from the benefits and a few quid more. So I work for nothing. There is also no consideration for travel costs or work attire or any other expense there. I work in a school so I don't even need child care. Most parents wouldn't be that lucky or have children too young for school. This country is broken for working families and the only reason I work is because I enjoy my job, I want to set a good example, I want a pension and my children are starting to leave the nest so benefits would go down anyway.

Literally who would ever want to do a job they don't like while someone else's raises their children to end up worse off? It's stupid x

Is there a reason why she’s not contributing to the family income in some way?

She IS contributing to household income by being the primary childcarer at home. She is saving them thousands.

I don’t think you say how far away this is, whether there are special circumstances, such as mother’s health or emotional state eg whether she is lonely.
Perhaps your DIL likes someone making a fuss of her!
Were your DIL to work could they afford the added expense of childcare to make it worthwhile - would she be able to earn a good salary? Nurseries are fiendishly expensive.
So many imponderables before anybody can express any sort of valid opinion.

She’s his mum. I think it’s lovely to be with your child in those first years. We were horribly poor, no holidays, no phone, certainly no eating out, clothes only as presents, but it was worth going without to have that time. It never comes again.

My parents lived 200 miles away and I visited about every six weeks for about a week. Not worth making the journey for less

Honestly I don’t really see what you’re so bothered about.

Unless your son is complaining then keep out of it. Actually, even if your son is complaining keep out of it as it’s none of your business!

FWIW I do think both parents should work wherever possible - it’s so important for women especially to remain financially independent as much as possible and paying towards a pension (as men do). Childcare is a joint expense so not something that the woman should shoulder on her own. Is there a reason why she’s not contributing to the family income in some way? Could she work her hours around his?

Only they can sort it out between them though.

May I ask why it’s your business unless of course your son has come to you complaining , if you live nearby enough to know how they spend their days why is it wrong for her to visit her family once a month to catch up with them Unless your son has asked her not to and I can’t see why he would then what’s wrong surely it’s right for the little grandson to have contact with both sets of grandparents
I don’t really see a problem If your son can not manage on the money he’s getting and you can do the childcare free of charge (to give them a good start) then hopefully she would like to get back to work