(NSFW) The sub who was really humiliated, what would you advise hime to do about it?

Say you have this friend who met this girl they are both into B&D; though she is the dominant most of the time while he was the sub. For about 3 months and change things when OK then he said one night after she had had him bound down she put on a strap-on and against his protest buggered him good. Now he is all bumped in the head, he wants to do something but what? He doesn’t know if he has any recourse at all? Most of the time he is fuming about her and what happened. Was it rape? If it was how can he prove that seeing he often gets tied up or bound doing sex, whipped and otherwise humiliated? Can she be sued for something? Should he just chalk it up to a lesson learned? What would you tell him?