I personally wouldn’t message the wife tbh, because at that point it’s guaranteed to blow up in your face and tbh this guy sounds like a master manipulator so I have no doubt that he will probably be able to convince both his wife and your mate that you are just some lying jealous hater who is trying to ruin things.
I had a similar situation with a good friend a few years ago (it wasn’t as bad as your situation, the guy she was seeing didn’t have a wife but he was a fucking bellend who had zero respect for her) and I agree, it does get so jarring hearing your friend constantly making excuses for him and convincing herself of shit like ‘oh he just has commitment issues’, ‘he just needs to get himself into the right head space and then he’ll finally commit to me’ - rather than just accepting the truth of the matter, which is that the guy is fucking trash.
If I were you I would get your other friends and try to just hold an intervention with this girl, it may be uncomfortable but sometimes people need to have people who genuinely care about them sit down and just give them a bit of a reality check.
If at that point she doesn’t want to listen, I would honestly take a step back from it all until she comes to her senses. It sounds really harsh but I found that with my friend, I was sort of inadvertantly enabling her shitty relationship because every time I picked up the pieces, once I’d comforted her and she felt better, she’d just go straight back to him.
Once I stopped engaging with her and listening to complaints about his behaviour and immediately comforting her when she was angry or upset with him, she sort of came to her senses and realized that the relationship was not normal at all and finally broke it off altogether.
It’s a frustrating situation to have to go through with a friend tho, hope you’re doing ok!