Gossip Girl

B is involved in a infidelity with A w/o M awareness.

Oh, B we all knew you wanted to do it, But why do it to one of your closes friends?
Payback maybe or just bored. Who's to blame? who knows their all both caught in a huge love triangle. M not keeping a tight enough leash on A or A needing to keep it in his pants considering he has a GF! Word is that B never actually liked A but completely loved R. Does M know or she just doesn't want to do anything about is B faking the whole "No, you completely got it all wrong" act.

I'll leave it for you to decide.