A friend in a similar position, said nothing, did nothing, just acted in a warm and friendly way to both but quietly undermined him, encouraged her DD to act independently, when she wanted to train for a profession, in your case you could express sympathy for his families problems etc etc.
They were working on the assumption that showing any concerns about the relationship could be used by him to encourage their daughter to turn against them.
It was a very thin tight rope they trod, but it worked, their daughter began to see a few niggles herself and her parent's support began to worry her because she could see the problems - and it worked, she saw through him, the relationship broke down and she was so grateful to her parents for all the support she had given her and her ex.
At the same time another friend just barged in heavy handed, DD promptly got married to the boyfriend and had a miserable 5 year marriage,