It sounds like that lady is doing something or has done something to either spook him or cause him pain in those two locations. There is something she is doing that you were and are not. I would say get an experienced trainer to watch her ride him becuse someone will get hurt that way. I didn't see it but maybe she has so much fear she conveys it to him and he wants her off, and since you don't he responds and trusts you. She might be poking him with her heel or snatching at his mouth when she gets scared, I don't know but if you were able to ride him fine, the problem is her, not him.