It would depend on how I found out. If I found out from her I would confront the guy hitting on her. If I found out through a third party I would confront my gf about it then proceed from there.
i'd tell him to stop.. if he continue well i punch him.....
put her in restraints
taser...the both of them because obviously the gf/bf isn't capable of handling the issue
haha..I did that before..I punched my ex's face infront of the girl lol