Talk about whatever, respecting the rules established by Lemmygrad. Failing to comply with the rules will grant you a few warnings, insisting on breaking them will grant you a beautiful shiny banwall.
A community for comrades to chat and talk about whatever doesn't fit other communities
Get them all together, tell them whats up with this person and ask what they are going to do about it.
Having experienced this in the past the correct response is for them to say 'fuck off' to the person then kick the fucking shit out of them if its anything but 'ok fair enough bye!'
If they wont do that they arent punks worth hanging out with, find cooler punks.
This is the only answer, paradox of tolerance tells me the abusive person needs to get booted out yesterday
to try and give a useful answer this time:
the reality is that if you, a (relative) outsider, try and come between a group of people and the abuser they consider a good friend, you are very likely going to lose. you can try to force the issue but ime you are going to see them close ranks and possibly freeze you out entirely, depending on the level of control their group has over the scene. none of this is to say "don't try," just that you should prepare for things going poorly.
an anecdote: when i was a kid my friend group was the heart of the party scene in our town. two of them ended up being pedos. no social consequences.
i think it's good of you to try, mind.