This sounds very sad, fishing4life. You sound like a good person supporting your wife in a difficult place. Some people here will give you a hard time and tell you everything you and your wife did wrong. But maybe it hasn't happened to them yet or maybe they just enjoy telling other people off.
When your children grow up, the things they have the absolute right to choose to do can hurt you and other people in their family. Yes it can feel like your heart is broken, especially if you did not see it coming. Sounds like your wife especially is grieving the relationship and future with your son she thought she had. That's a big loss but one day she will come out of that pain - we always do. She will stop holding onto an image of a career and a relationship that have gone. Everyone else will be happy in what they are doing - because people do make the best of things - and she'll see she's the only one working to hold on to the hurt. Then perhaps she'll want a bridge back to the son she actually has. Maybe you being in contact with dil and the gc can be that bridge. Its not betraying her, its looking out for her in the future.
(I am speaking from my experience - dp and I thought celebrating our daughter's wedding to her long-term partner was the most romantic and happiest day of all our lives - turned out 2 years later the whole time she was having an affair with the person who is her new partner and now father of our gc. It was a complete shock that I still feel 50 times a day. But I have got used to my new feelings about her. I love her to bits, but I wouldn't again allow my happiness to depend on what she does - and indeed why should I or she have that responsibility. And although I would still rather her current partner had never existed, he isn't the marriage-wrecking slimeball of my imagination either. He's actually perfectly nice!)
I say that to give you an example that things can work out.
Things will never be the same again but they don't have to be bad. And yes it is worth being grateful that your dil does keep trying.
Hope you can all put it behind you and find happiness. You don't have to do anything you don't feel like, but maybe one day you will want to and you will be pleased you haven't acted to slam the door shut.