My advice is to just fade away slowly. You can't do anything to stop their behavior, and you're right that it would only make you a target. The one nice thing about narcissists is that they don't really notice what other people are doing. Get involved in other parts of the fandom. Get known for something other than friend-of-bnf. Don't defend him/her publicly, but don't trash him/her either. That's what I did and it worked pretty well, actually. I would say that we're merely friendly acquaintances now, and that's much better for my sanity.

I wish we didn't have to be anon! I could say so much more but people would definitely work out who I was friends with. (Also? That post of the BNF sounds awesome in a completely ridiculous way.)