How do you break up a couple
Instead of finding a way to break them up, I suggest you talk to her if you know her. Tell her how you felt & be upfront. I felt sorry for the guy though!
Frame the two for cheating on each other, spread rumors, come on to one of them, etc. but why would you want to ruin a relationship
Spread rumors, make the guy/girl like you more stuff like that
Tell him that you saw her flirting with her ex again
Same on you say there cheating. (Evil laugh) ;)