As he is perfectly happy with her all day, I don't think it's a major issue but you hint that, aside from this, she is not feeling that brilliant at the moment so possibly this is why it is affecting her so much. It would be a lot worse, I feel, if your grandson had a poor relationship with his dad and didn't interact with him when he returned home.
Has your daughter got some supportive friends who she can meet up with? I think there are a lot of pressures on new mums (or dads) at home. They can become isolated and introspective and lose confidence and it's important to get away from what can seem like an endless round of domestic chores and have a few laughs - and moans - with friends.