OOoooo... the nerve of something people.
I think your friend should make this other guy pay! I'm usually a nice, forgiving guy, but this is one clear opportunity (I think - I don't know, not a legal expert or anything 😉) where something very nasty can be done, without bad consequences for you. Well, maybe the other guy can leave negative feedback or something? I dunno... but still...
My vote: a huge, hi-quality image of something nasty 😀 There's plenty of pr0n available, sure, but his picture host may have restrictions against adult material. But I'm sure you can find nice disgusting, non-pr0n pictures as well!
<-- Can't wait to see what happens 🙂