Songs about knowing someones cheating on their significant other?

And also give me advice please. My best friend is dating this boy, and i'm dating his best friend. Sounds perfect right? Except for that my boyfriend knows that my friends boyfriend is cheating on her. So its either ruin my relationship with him and keep my friend blissfully unaware, or tell her and destroy her world and destroy my relationship with my boyfriend. Are there any songs about anything remotely like this?