my son has left his wife

I think your son has to be honest with his wife.
Tell him you can't stand lying to her and that you feel you will have to tell her if he won't do it.
If he doesn't think their marriage is over, why is he seeing someone else?

If he doesn't want you to be involved in what he does, and what he says to his wife, he shouldn't be living in your home.
Maybe a suggestion that he find somewhere else to live could focus his mind on being straight with everyone.

In a similar situation I simply said that I wasn't happy with the situation, that I was not ready to meet the new 'friend' and that he/she would not be welcome in my home.
If you mean he has moved home to your home he has no right to impose this situation on you. Is he bringing her to your home?
Your house your rules.
Rightly or warongly, I'd be tempted to say how unhappy yon are about feeling disloyal to your DiL and that if he doesn't discuss things with her then you will.

I agree with Farnorth , it’s not your secret to tell and I suspect your DiL would shoot the messenger!

However, you are enabling your son , so I would have a conversation with him and tell him you’re not lying for him. Put the responsibility straight back on him .

For what it’s worth , if he were my son , I wouldn’t have made it so easy for him to leave his wife by offering a refuge. I know you love him but it’s so easy to move back home with Mum. If your escape had not been there , might he have stayed at home ? Worked a bit harder at his marriage? Not found it so easy to cheat?

I don’t know but I really feel for you in the situation you’re in .

I agree with FarNorth.
Are the DC very young? Children notice things and if you don't tell their mum she'll find out soon enough.

I haven't been in this situation but I would keep quiet unless my DIL asked a direct question. I wouldn't lie. I would explain that to my son. I think you just have to give him time and see where that leads. Of course you are sad and I feel for you.

I wouldn't have taken him in under those circumstances. It's so mean to put you under this obligation because he has another woman. The DiL will now think that you condone this behaviour and are party to it.

I'd have sent him right back to sort it all out between them.

Basically he is using you and not respecting you. He is an adult and needs to get on with it.

You must have quite a big house which is lucky.

Thank you all for your advice. Initially my daughter in law asked me to let my son come to my home. She was very worried about him and wanted me to be there for him. He had had mental issues in the past and she was worried because of his behaviour. It is totally out of character for him to behave like this. I told my son since learning about this other woman that I'm really not happy so he said he would leave. I told him the real problem is that he isn't being honest with his wife and if he doesn't tell her I will. He's aware of this now but I still feel it might not be right. I'm torn.

You don't say what was the original reason. or explanation for the split. As a completely detached person. it seems to me that there is a strong possibility that this 'other woman' was already in the picture before the split and that your DIL may know this but is choosing to turn a blind eye in hopes that it will fizzle out and your son will return to his family.

" The DiL will now think that you condone this behaviour and are party to it."
Good point EllanV.
Grannibobs - if your son wants to leave his wife and family he needs to find a place of his own to live. Is he working?
It's a very sad situation, but it's their problem, not yours.
I have a close friend who is going through the same thing with her son, though he doesn't live with her. She's so upset, it has changed her.

I agree with what you say. I've told him in no uncertain terms how wrong he is. I don't want to know about this woman. I've told him numerous times to sort things out. My daughter in law doesn't deserve this.

An important issue for you must be seeing the grandchildren. If you fall foul of your DIL by keeping this secret then you may put this in jeopardy.

However, I am not suggesting that you tell her, simply that you continue to be clear with your son that you are not going to collude with him in this - and that he must tell her what is going on. She is likely to be understandably furious when she finds out and knew that you were aware all along.

I don't know what the reason for leaving was. My son and daughter in law both said they had been having problems for a while but didn't really go into detail. If I'd known he was going to go off with someone else I wouldn't have made him feel welcome in my home.

I don't agree with the people who say that you shouldn't have let your son stay with you. He's your son, of course he comes to you if his marriage isn't working out. All this talk of, if you hadn't made it so easy for him to leave, they might have worked it out is rubbish.
If he has fallen out of love with his wife, nothing is going to get them back together.
I don't think that you should tell your Dil about the woman he is seeing but I would tell your son that you don't want to be involved with this woman in any way, until he does tell his wife and whatever you do, don't have her in your home. Be very firm about that.
When he goes out to see her, tell him that you don't want to know,
as it's not fair on you and making you feel disloyal to your Dil.
Then treat him as a house guest and don't get involved with his private life so much.
Look after your Dil and Grandchildren and just do your best, that's all you can do.

There's a fine line between lying and not telling the whole truth!
I do feel for you and think you've handled it as well as possible so far. You clearly thought you were cooperating with your DiL by taking him in and it's sad that your good intentions seem to have backfired.
I agree that you must stop enabling him and let him deal with it.

Sorry X post, I see that he is now moving out.
I still wouldn't interfere in their business and tell the Dil.
He should tell her but that is up to him, not you.

Its a catch 22 situation Grannibobs either way your daughter in law is going to be upset, if you tell her it may be shoot the messenger if you don't tell her you are colluding with your son.
I understand you offering a bolt hole to your son but he was clearly not honest with you, I think you should insist he tells his wife the truth or finds somewhere else to live. Sometimes we have set boundaries even if our children are adults.
If there is a mental health issue here then he needs to get help for that.

I do admire the way one of my neighbour's MIL has behaved. Her son left my neighbour yet his mother still frequently goes round to do the school run etc. and is obviously still good friends with her ex DIL. Her son was banned from entering his old home so when he comes to collect his children, his mother keeps him on the doorstep!

Eventually my neighbour met another man and her ex MIL seems to have made friends with him too!

The two words which say it all are BACK HOME.
Home, to your son, is the place he shared with his wife and children.
He's moved into YOUR home which stopped being his when he launched his adult life and got married.
I think it's wrong to keep schtum about his 'other woman'.
Your DIL will be mighty cross if she thinks you've been in cahoots with him about his other relationship.
AC shouldn't bounce back to mum and dad when things go wrong.
Let him sort out his life on his own without facilitating clandestine relationships, otherwise your DiL could start limiting your time with the GC.
I bet the OP is cooking, cleaning and doing his laundry.
My advice? Chuck him out.

There seem to be increasing numbers of people with mental health issues.
On almost every thread, somewhere, you'll find that someone has blamed or excused behaviour on a person they know having mental health issues.
Why can't people cope any more? We have it easy compared to yesteryear.

The fact that the son in question has had mental health issues in the past probably put his mother on the back foot here.

Maybe he wants the easy option which is you telling his wife for him? Men can be cowards

Perhaps DIL guessed that there was someone else in his life and asked for him to be allowed home with you rather than give him the option of moving in with her.