My friend is cheating on her fiancé with more than one guy. Should I react?

This is actually pretty sad but my best friend I've known for very long is in rehab. She had ADHD and when she was young the doctors prescribed her too much ritalin, but because of a medical blunder, she eventually got heavily addicted to ritalin and other calming drugs so her fiancé put her in rehab. It's really sad because she is such an amazing girl and because of someone else's fault she get's locked up in a rehab center with lowlifes. I got to visit her for a couple days (friends can help after a while) and I was shocked by how things were. She has an extremely hard time but a worker told me she was doing amazing and is a role model for the rest, but the circumstances to me were really fucked up since I'm not used to that kind of environment. There is a rule that male and female patients can spend very limited time together and sex and relationships (except for already existing ones) are forbidden because if it goes wrong, they will probably relapse. Of course there aren't enough workers to keep everything under control, so a couple guys sneak out and fuck some women every day. During rehab everyone is really horny and sex tends to distress. She has been fucking 5 guys alternately. No love involved, just fucking like monkeys and they part. I even walked in on her and she had a guy behind her who just released his load on her ass, but they didn't make a big deal of it. It's just a couple guys who have sex constantly with about half the women there! I know I can't imagine what she's going through and of course in that environment cheating is not that bad, but I never thought she'd do that. She is very attractive but has always been very loyal. I feel bad for her fiance though but I don't think I should tell anything. She has been there for me and had my back so many times. I really don't want to betray her. Even if her fiance doesn't know the truth, I don't think it would help anyone. Maybe if she is stable again, she'll talk about it. But what do you guys think?
My friend is cheating on her fiancé with more than one guy. Should I react?
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