Your friend is stuck in a crappy situation, which is party her fault, and she’s making it your problem also.
But you’ve got a couple of questions to answer before you can make a plan. Maybe you feel a little resentful because you’re not dating anyone right now.
You know your best friend inside and out, and you never would have thought she’d go for someone this pathetic.
Every time you have to hang out with them you pretty much have to staple your mouth shut so you don’t scream “ARE YOU SMOKING CRACK?
What I've learned about friends dating idiots, the idiot is perfectly capable of severing ties on his own, whether it's cheating, hitting bottom on drugs, or worse. High school girls love bad boys, but after graduating they usually want better than a professional cannabis taster.
Graduation is also when you learn who your true friends are, which is why I recommend sticking with her.