Sex with an ex's friend?

My ex and I have a complicated relationship, we are friends, then we hate eachother, then we patch it up the the cycle just keeps going; it's almost been a year, we were friends with benefits in secret for a few months 6 months after we broke up because we sort of had feelings for eachother but we couldn't date because of other reasons, his mate, not his best friend but they hang out sometimes wants to get with me for just a one time thing and I want to as well because he is soooo hot and I am at the stage where I dislike my ex right now because he fucked me over big time so I think this might piss him off because he has this other girl (she's kinda a slut) but apparently he's using her to get over me (that's what his best friend said to me (his best friend and I are really close too)). Should I do this or is it a bad decision?!?! (Keep in mind I am in highschool so this is teen drama don't judge hahaha)