So somebody touched my girlfriends thigh what should I do?

So there's this guy me and my friend recently met he seemed like a good guy but we eventually found out he had a seizure problem and his parents had kinda kept him sheltered because of his issue thus he was highly socially awkward it wasn't a problem at first until we realized how much of a pervert he was he had touched many of our female friends in a sexual way and it made us both fairly angry but my last straw was recently pulled when he touched my girlfriends thigh I was deeply angered by this and she has already asked him to stop I want to react to this but I don't know how to any tips?

So somebody touched my girlfriends thigh what should I do?
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