Before reporting him, tell him- in a public place, possibly the lobby or most popular place in the convention center- that your friend isn't interested. Yes, he's being creepy, but if he doesn't know that he's being creepy, or doesn't have any ill intentions, than it's not really fair. If it continues, then report him.
Upcoming Conventions: Katsucon, Anime Detour, Wizard World Minneapolis, C2E2
Ino Yamanaka- Naruto Stage Play Live
Numa- Star Wars Rebels
Rachel Amber- Life is Strange