I left my marriage of 24 years ( not for a younger man though!) and my then mother-in-law was as devastated as you are now.
She never forgave me (she died last year, 24 years later) and I am still sad that we could not be reconciled as I thought the world of her.
To her credit, she never criticised me to the children but sadly, did not talk about me either regarding happier times - it was as though I was dead and the children did find it very difficult to understand how she could hold onto her silence and ignore the past. We met at family occasions and it was civilised, but she could have enjoyed so many more family times with us, if she had relented and forgiven the past.
So, however devastated your son is and you are too, please try and just be there for him and the children as you say, but also, one day, you may wish to reconcile with your daughter in law.....please leave the door open......life is far too short to prolong unhappiness and misunderstandings.
On a practical note, get your son to take legal advice immediately regarding a formal separation and get the couple talking together, perhaps with a mediator, as it will save so much more aggravation in the future and a sensible compromise regarding property etc. can be reached earlier rather than later.