DIL disappearrs to her parents all the time

How often SHOULD she see her parents? How often is she allowed (by you?)
If we settle for once a week (seems reasonable) thats 4 times a month.
Your DIL would see her's 12 times in 3 months if she visited once a week.

But Dil's parents live some distance away, so having gone to the expense of travel it seems reasonable to stay. So she stays 5 days in 3 months.

Problem with that?

How often does your son see you? Quite frequently from the sound of it.

The work or not to work problem is completely separate .

No - she stays with them for five days every three weeks, not every three months.

I read this post with interest, OP says MIL comes every week
yet dil still does to stay every 3 weeks, which sounds like it is causing a problem.I am a dil and i would not dream of staying with my parents if i show them every week specially if finances were tight.

Basically, DIL is spending about one week a month w/ her parents (ok, 5 days but that's almost a week). That says to me that either she misses her family very much, she needs more help w/ childcare than DS has time to give (I bet they watch baby a lot while she gets to nap, etc.), or there are tensions in the marriage she feels the need to get away from. No matter how much she "enjoys" her parents, I can't see leaving a husband and taking your mutual child w/ you (general) this much if there weren't some kind of problem.

That said, I have to agree, meet, these are not your issues to solve. Not the issue, if any, about the visits, the one about her not working, or the one about what she spends on what. All these are things DS and DIL need to work out between themselves.

It was nice of you to offer to watch GC a couple of days a week, so DIL could work. But it might be more helpful if you offered to watch baby to just give her a break and a chance to shop, relax, whatever she needs. That might prompt her to cut back on the frequent trips to her parents.
But you may not want to babysit for those reasons, and she might still not accept it.

Beyond that, I agree w/ the others that you need to stay out of it. Next time DS complains, in fact, I would (gently) tell him that he needs to work these issues out w/ DIL. If he persists in venting, be sympathetic but for your own sanity, change the subject. Also, please remember that as upset as he gets, he, no doubts, makes up w/ DIL when he's w/ her. Let that thought put things in perspective and don't let yourself get too worked up over these concerns. xx

No, 5 days every three weeks on top of seeing her family throughout the week. She’s only living with in the family home for a fortnight at a time before she moves out for five days, and she sees her parents regularly during those two weeks.

I’m not sure how I’d have felt if I’d been the working parent when our DC were little and DH had done that to me.

How far away does her parents live? Is it possible for them to move closer to parents?
Agree with those saying you should re-direct your son to speak to his wife instead of you.
She may be finding it difficult to be away from close family with a young toddler. Perhaps she feels isolated where she is. Maybe your son can encourage her to join a club one evening during the week so she can make more friends locally.
I read it that she spends 3 weeks at home then 5 days at her parents.

Yes, on re-reading you could be right craftergran . Either way, if she’s away that often on top of regular meetings throughout the week with her parents (which suggests they don’t live that far away) she’s already seeing them a lot for support - far more than many other young parents.

Does DS have long hours and she is off to visit mum? DS should tell her not to you if he doen't like these visits.