From the sound of your description, it sounds like there is a course set out in the ring everyone uses that’s not moved or lowered, and she was riding at the same time as these other people, and perhaps followed their lead. It doesn’t make it right, but it doesn’t sound like this was something done on her own initiative. A bit different, psychologically speaking, than setting up the course herself and cowboying about to see how much she could push your horse. Some teens are very timid riders; don’t assume every single teen is a limits-pusher (although they certainly exist in spades).
I would also add that I would make absolutely certain that the story wasn’t exaggerated secondhand. I don’t want to start a big leaser vs. owner debate, but I have on two separate occasions when leasing a horse being accused of doing things I simply did not do. I don’t mean “well, it was open to interpretation, but I don’t think it was that bad” but flat-out, did not do it, yet it was reported to the owners in no uncertain terms that I had. Yet once an owner has that niggling doubt about a leaser riding a horse too hard, it never goes away, and the horse so much as sneezes after the lease ride, the owner is inclined to be suspicious.
And I understand, because horses are expensive, unpredictable, and it’s scary to lease a horse. But as well as everything else, I would ask the young woman for her version of events, and then clarify you don’t want any jumping outside of lessons except for crossrails and poles.