Daughter and Son in Law dilema

My daughter and son in law have been married for nearly 8yrs. Although I knew things were not great between them, I put it down to them being in a relationship for 16yrs and two young children as well as trying to juggle working in very demanding jobs. He left her just before Christmas but he only stayed away for one day when he came back. I thought they would sort everything out but just after Christmas he left again. He's adamant that he's not coming back this time, although he calls in most days, usually using their children as a reason to call. The children are struggling although they appear happy when he is around. My daughter is distraught and has been diagnosed with depression. She longs for his visits and is devastated once again when he has gone. He swears there is no one else involved and is living with his parents at the moment. I don't know what to do to support both my daughter and grandchildren. Any advice would be great.