Was there ever a 'bill of sale' or any kind of contract that stated rules/limitations for the girl, the 'adopter'? If not, there is no proof that the horse truly changed ownership. I am not very keen on the specifics, but when I was going to do a lease to buy, when the sale was going to happen, i'd get the horse for 1 dollar just to make it a legal sale. However, the horse was NOT legally mine until I bought said horse, and everything had to be oked by the real owner- even when I opted for specific tests by a vet for the horse-which also would need an ok to move barns ( which wouldn't have been allowed) Also, was this more of a full lease situation/on site lease where she was paying board and upkeep of the horse? Unless paperwork was signed as legal ownership was turned over to your friend, the way I see it- the horse still belonged to the original owner. ( I adopted from a rescue, my horse legally belongs to the rescue and always will- I just get a 'free off site lease' of the horse)
I'd say your 14 year old friend had a hard lesson to suddenly learn: unless there was signed paperwork stating she was now the official owner-ownership never changed hands. If a situation arises like this again for this friend, or you or anyone reading this post- make sure the terms are crystal clear and fully agreed upon.
However, hoping things can be ended on decent turns, your friend can always let the owner know that if she no longer wants the horse or cannot care for it, she'd be willing to buy the horse.
I am terribly sorry for the heart ache the situation caused.