First off, 15 year olds doing anything approaching illegal is never going to be cool. Not sure what kind of girl Susan is, but you said she was peer pressured into participating in such shoddy business, so as least it isn't as bad as it could be. Is it a first time offence? The biggest problem you need to address is Susan's shitty 'best friend'- these kids are a dime a dozen juveniles resorting to cheap thrills to either pose as badasses or get away from their terrible life problems like parents and tumblr updates.
Jack need to show Susan that these are not the people she needs to associate with- you're all pretty young, you don't need to keep up friendships with bad company. There will always be opportunities to make new friends and socialise with better people, no matter how large or small your town may be. This will be far more enriching and beneficial long term for Susan than fearing any so called friend at this moment might not like her on the basis of weed and alcohol. If Jack can't appeal to her morals then he should point out just how incredibly BAD this stuff is, especially during this age period. Tell her about brain damage. Seriously. Play that card, because it's a real possibility.
Anyone can google the adverse effects of weed on an adolescent brain, and it can be detrimental to a person's future. We're talking serious emotional and cognitive problems. And alcohol? Yeah that's gonna screw you over in the long term, too. Show her as well as tell her what these things can do. Sometimes fear is a great persuasive tool.
Finally, tell Jack he's doing the right thing by her. The dating scene at your age is pretty crazy, I sure as hell don't want to be back there again- but Susan has a guy who legitimately cares for her well being, and that's a beautiful and rare thing to come by. If Jack is just as important to her, then hopefully Susan should at least take his concerns into account, because if this keeps happening then it will keep affecting the both of them. Ultimately, Susan is going to be worse off in the longer run.
Ideally Jack needs to find a time, sooner than later, to to tell her how he feels. He need to be calm, and not get upset, even if Susan starts to. Hoping this all gets worked out soon.
Best of luck to all of you!