If my BF cheated on my I would trick them into admitting it: Then make him Introduce me to the gal (or guy) and we can team up and take him down.
I would cut his hand off and throw him in a tank full of hungry sharks and then when he's about to die I would make the girl who he cheated on me with stab him than shoot herself!!!! In other words I'd try and punch and kick and slap them or at least try to