If either person does anything outside the bounds of the session -- i.e., rape, beat, mutilate, blackmail -- it wouldn't be unreasonable for the victim to warn the community and, if the situation is severe enough, notify the authorities, though the victim might also want to get the advice of legal counsel beforehand. Some people might advise backchanneling such information through the community, but again, I think that would depend on the individuals involved, as well as the severity and the frequency of the situation.