So sorry, granny50. But I agree with PPs (previous posters) - just listen and maybe utter a few sympathetic replies ("That's too bad," "Poor darling!" etc.) It's hard for us moms not to say anything, I know - we feel as much for our DDs and DSs when they're adults, as when they were kids - that part of being a parent never ends. But as Luckylegs said, this is between the two of them and you can't be the "third party."
Besides, what if she takes your advice and it backfires in some way? Or as Pentstemmon says, what if you bash him and then they reconcile? Better to bite your tongue and just be a sounding board. If you feel the situation is getting really bad, maybe suggest counseling - once - and then leave it to them - but that's all. And maybe not even that.
(((Hugs)))