[GameMaster]

It depends on whether you're friends with her at all or not. If you're not close to her, I'd consider telling both guys, since they're the ones being violated/cheated. She's the one who's at fault.

If you're friends with her, I'd ask her if she told both guys that she was seeing another person at the same time. If she didn't, I think you should advice her to tell both guys that she's involved with multiple people. They need to be informed of what kind of relationship they're involved in. If she doesn't tell them, you need to.