I think I need some personal advice

[John Dibble]

If doing nothing wouldn't significantly bother you, then do nothing and let the situation play out.

If for whatever reason doing nothing would keep you up at night, then I'd talk to your female friend directly and let her know it bothers you. Let her know that you think she should either stop screwing around or break up her current relationship, because unless she's in an open relationship she's only going to end up hurting the person she's with now. Let her know after that you won't mention it again because you only wanted to get it off your chest, and that you aren't trying to dictate how she should live her life but that you just want to give her advise as a friend who respects her. Depending on her this could be taken well or be taken poorly, but again if you have to do something I think it's more respectful to talk to her directly rather than going around her back.