I”m curious why you think she should stay??  Although you call it a ‘perfect marriage’s it clearly isn’t.

Big houses and a history count for nothing if you aren’t happy and it doesn’t sound like your friend is happy with the situation, unsurprisingly.

I’d suggest being more supportive when you listen to her, she’s probably scared of making such a big change and standing up for her self. Doesn’t have to mean leaving but she really does need your help for the strength to repair this relationship, if the guy is even interested in doing so.