I think your friend should lease a different horse unless she's just going to ride this one to work on her riding skills and not worry about the rest. If everyone is not going to handle and ride the horse in the same way, then you can't expect much progress in its knowledge. Whatever you end up teaching it, the next rider is probably unteaching.
It's nuts to have that many leasers for one horse, IMO.
* I'm often reading and posting from mobile and Siri loves to make a mockery of the English language.