Is my friend a perv?

Exactly. I wouldn’t classify a hickey as a criminal offense. Now, if things get a bit more intimate, then a criminal offense could be committed. I’m not saying that the 21 year old sucking on a 15 year old’s neck is nothing to be concerned with, but it’s not criminal.

Since she’s your friend, why don’t you talk to her about it?

i’m posting this blindly (without looking at the other responses. yet.), and i’m guessing i’m in the minority, but i feel that nothing is wrong with that. not for me, as i chronically persue older men, but it seems fine. six years difference. big whoop.