My best friends girlfriend grabbed my ***? what to do?

My best friends girlfriend grabbed my *** like literally grabbed it, while my boyfriend went to use the washroom, usually me and her don't talk to much, but i took her pencil case and she was chasing me and then she grabbed my ***... my friend is really strict on her and he doesn't allow his gf to do these types of things and she never has before, should i tell him, i don't want this to affect their relationship. plus his gf is really shy i don't know what happened its shocking. what should i do?