I've been put in this situation many a time with my former band mates. Not fun...but I think in the end (and in a nutshell) the "answer" lies within yourself and how you feel more comfortable living after you've made your decision.
Not saying that my answer is the correct/wrong choice, but I kept my mouth shut. I would rather be pulled into the situation and asked, than seem like I'm intentionally trying to stir the pot. The musicians I know, would be hurt that I say something, insinuate I'm trying to bream them up and sleep with her, and you're branded a snitch. Yet they are the ones that are being unfaithful when it's agreed upon that they are monogamous.
If he/she is breaking an agreement, then I think the person will have to live with themselves and their decision more than you doing what you feel is right. You don't have to fall asleep with that person...only yourself.