Last year I was in an awful situation with a long time friend of mine and his batshit crazy fiancee and let me warn you OP...you think your friend will realize his partner is being crazy but when she made him choose between his old friends (me and my bf) and her he chose her. She clearly had some jealousy issues (we were completely platonic, respectful of all boundaries, and 99% of the time hung out in a group) and I think she finally broke him down by fighting with him about his friendship with me all the time. Despite the fact that i made a point to be very nice to her every time I saw her. I really hate remembering what happened because we saw the warning signs early of her being controlling and didn't say anything to him about it. I regret not trying to give him a heads up before she made him feel like he's in too deep to quit. Your friend sounds like he's in a similar situation which means you might have to brace yourself for the worst, that he appeases her to try and salvage their relationship. Hopefully he will come to his senses instead but this girl sounds pretty set on ruining everything in her path.