Need some drama for a story I'm working on and which would be worse?

Best friend is worse. Also, in short order, that best friend becomes his worst enemy. That kind of betrayal, that's how worst enemies happen.

The reason sleeping with his best friend is worse is in one fell swoop he loses the two people who are most important to him: his girlfriend and his best friend. His social circle suddenly shrinks drastically. He's left all alone and the people he'd usually go to for support are the people who have hurt him, so that "all alone" turns into "lonely as f*ck," straight up despair.

If she sleeps with his enemy, sure, that'll piss him off, but that guy was already his enemy, so she's not really creating any additional loss for him. When he hears about it, he's going to call up his best friend, they're going to go out drinking or whatever, they're going to talk sh*t about her and that enemy, and he's going to already start to feel better because he's got his best friend giving him emotional support and having his back, so maybe everything will be okay -- bros before hos and all that.

That's why sleeping with the best friend is worse. Take away a guy's ho AND his bro? Ouch! That's not double hurt. That's hurt squared.

how about both.. and her friend gets tied up in it?

probably her ex's best friend