Ask ur son to talk to his wife, about his past mistakes and how much he regrets having hurt her. Ask him to ask for forgiveness even if he has done it before. Ask him to lovingly sit beside her for sometime and remind her they loved each other once upon a time and that he still loves her and is sorry for his actions.
Ask him to do this lovingly and geniunely. And yes before he talks to ur daughter in law ask him to talk to God. Let him ask for forgiveness from God once again.
I'm sure your daughter in law loves ur son but rite now she is very depressed and is feeling hopeless. So true love and genuine words of repentance will definitely bring them together, m very postive and sure about it.