OHhh...for some reason when I read your post, I read it as she IS acting weird and in pain. I don't know why I read it that way. You said just the opposite. That's why I went into a whole bunch of questions.
Anyway, moment of my own insanity has passed.
Rexes can be susceptible to bald or thin spots in certain areas. Wherever their fur is thin already, can end up being a problem. That's why sore hocks can arise in a Rex even if they have soft flooring.
If it doesn't look red, just bald, then it could be just from how she sits and positions herself. She also may not like anything on her rear and like your friend said, and just by cleaning so much, and rubbed the fur off. Could she have sat on something that made her want to clean that area so much? Do you see her cleaning that area? if not, again it could be just how she positions herself, and if she's in middle of a molt (???) then this could happen. Many times bald spots will happen in several areas during molts.
Just keep an eye on it, be sure the area doesn't become inflamed or infected as bunny skin is very sensitive. And hopefully soon the hair will grow back.
And actually this probably ended up being a blessing in disguise. It is a good idea to research now and find an emergency vet that at least knows something about rabbits so that if a real emergency does happen in the future, you will have some place to go. I have found it difficult to find an emergency vet who is a rabbit expert, but finding an emergency vet that at least can help you in the future will be a blessing.
Keep us updated on your bunny's bum.