If somebody was doing that to my girlfriend I wouldnt touch him. I would pull him to the side and tell him that his childish games are disrespectful to me and my girl and that he needs to stop. But in your case she allows it so I would check her out. She wouldnt be worth my time if she lets guys touch her while shes with me because that doesnt seem very trust worthy to me.
Tell him to stop. If he doesnt, then get him on sexual assault... unless, of course, your girlfriend likes it, which seems to be the case. Ask her if she could tell him to stop. Or just beat the crap out of him, its your choice man.