Cheating

So me and my friend have been best friends since I was 4, she was 2. We literally grew up together! She stood with me at my wedding 2 years ago and recently asked me to be her matron of honor. I was so excited for her... until recently. The last 6-8 months he has been cheating on her, talking to other women, sending/receiving pictures, staying all hours of the night at womens houses "fixing their cars". She gets so upset and comes to me crying every time. I can't tell her what to do, that's her choice. I have told her my opinion, I dont think shes in a healthy relationship and she chose to ignore it. That's fine, that's her choice. Problem is, I'm having a really hard time supporting her upcoming marriage. Like I no longer care that they picked out flowers or invitations. I feel like shes in this unhealthy, unhappy, relationship and she deserves SO much more than that. I'm trying to fake my excitement and I know she can tell it isnt genuine. Idk what to do. I want to stand with her more than anything, but how do I do that if I dont support them?