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.
Next Convention: Anime Detour
Scarlet Witch- Avengers: Age of Ultron
Maggie Greene- The Walking Dead