"Be neutral about DIL in case they get back together"
This ^^
It might be a temporary break that simply looks permanent at the moment - who knows what the future really holds. It could just be that your DIL is finding her feet in a new job/circle of people, and has had her head momentarily turned by the excitement of it all... or it might be that the marriage is, sadly, over and that's that. You don't know if she'll get back with your son - or even if, once the shock has worn off, your son will even want her back! He might, once no longer blindsided by the whole thing, decide that if she can treat him so shabbily, he deserves much better.
What you do know, though, is that you now have to walk a very fine line. Support your son, be there for him, encourage him to seek counselling/go to his GP... but also without saying one negative word about your DIL, no matter how tempting it gets. Just in case they do get back together again. Because if they do, and you've been negative about her to him? It will be held against you by at least one of them... and possibly even your own son. It happens.
A friend of mine split with her husband just after the birth of their second child. He hadn't wanted the baby, there was a possibility that he'd found someone else, my friend sobbed for hours on everyone's shoulders. Her mother, did as mother's instinctively want to do... and said something about how abysmally her SIL had treated her daughter, their children together, and my friend's older child from an earlier relationship. Then my friend took her husband back. And cut her mother from their lives completely, because she knew (she said) that her mother had shown her true colours regarding the husband. Had my friend's mother walked the fine line of "oh, dear, it's terrible, and you need some help to process it" without saying "how dare that person treat my child like this!", it might have ended more happily.
The worst thing is that until the husband left my friend literally holding two very young babies, juggling her teenager at the same time, and trying to keep everything afloat whilst he "found himself"... her mother genuinely thought of him as the son she'd never had.
So be careful, OP. Support your son, but in a very neutral way. 