Backstory to not drip feed.
Me and DP have been together 5 yrs, mil (she will be my mil!) and her partner have been together 5.5 years. A couple of years ago they had a bit of a falling out, her partner shouted and swore at her (after drinking) in the street and stormed off to a friends house to sleep. Mil was distraught, confided all this in me and begged him to come back. After two days he did. DP was away at the time and mil begged me not to tell him, I told him the basics.
Then yesterday happened. I spent the day with mil. We ended up having a couple of drinks in the pub then back to hers where her partner had just finished work and my DP had too and was there to pick me up. Her partner seems quiet. He started talking about his weight, he's 15 stone and only 5'5". He then blamed mil for 'making him fat' because she 'piles his plate with veg' (??!!!). He actually snacks constantly until bedtime. He blamed her for buying chocolate (she'd stopped), for too many carbs (she gives him half a potato or equivalent) and the 'dreaded' four types of veg (steamed, no butter) which was 'obviously' wrong. He wouldn't listen to us and it was a half hour barrage of almost abuse at his mum. We tried chatting and making light of it, changing the subject but he would bring it back to her making him fat. We were embarrassed for her, shocked at him and we left. A couple of hours later she rang and asked me whether he was right about her making him fat, her partner had gone out, I told DP to go to hers and give her a hug. We both went. We chatted, told her it wasn't her fault it was his nightly feasts, everything was ok and he was probably just being grumpy. We were about to leave when he came in, mil had asked DP to have a word with partner so they went outside for a chat. DP told him that he thought he was a bit off with his mum and all hell broke loose. 'It's none of your fucking business', 'I can say what I fucking like' etc. he then stormed back out again.
She was upset hearing this, we stayed with her. She rang him, he ignored her. After three hours he came back in and went straight upstairs to bed ignoring us all. We went home.
DP now hates his mums partner. Wants an apology from him (it'd never happen), his mum was grateful we went but now thinks the family is split. If we hadn't gone over he wouldn't have stormed out. I've been awake all night feeling guilty for wanting DP to check on his mum. She, as of last night, wants me to chat to her partner and make things alright. I don't want to. It's an absolute fucked up situation.
I've been in an abusive relationship, so has DP. Her partners behaviour to her was setting off red flags. As her partner started off blaming her for his food DPs nails were dug into my hand. His mum will roll over backwards to make everything ok and never sticks up for herself.
Did we do the right thing?