Dude. If she's smoking pot at this age, she's bound to walk a path that will involve some shitty next few years filled with mistakes, especially if this creepy 20 year old is involved. I hope she wasn't pressed into fucking him already. Also you bet your fucking ass this shit happens.
Well, groups descend into this shit together. So your dad needs to sever her connections to these shitty brats, even if it means making her switch schools so she gets away from them during the day time and gets a chance to make new, better friends. It's THE ONLY solution that doesn't involve moving altogether because the more he tells her to stop seeing them the more she'll want to. It's how teenagers work and I guess that's what you mean by he's stuck on what to do.
Also pro tip, your dad should call the cops on these kids (including your sister). They're too young to be properly punished but it'll scare them and probably split the group into the "the ones who just went along" and "those with no future".