Direct communication between the parties would have been the way to go on this but that didn't happen. Hopefully with time the situation will resolve itself but I see no wrong doing on the part of the couple.
I also see no wrong doing on the part of the lessor.
Do you agree it's possible that the OP's wife overstated her horse experience? People do it all the time.
Regardless, it seems to me that there was quite a bit of miscommunication on the part of everyone
involved, lessor and lessee.
I have a 24 y/o horse with arthritis. I've retired him due to other issues not
related to his arthritis. At 19 y/o he was still being ridden, and didn't seem to have any problems.
Arthritis doesn't just suddenly crop up. It's a preexisting condition that won't necessarily affect the horse adversely, unless it's being over used. If the animal truly does have arthritis and its owner is aware of it, then yes, it should have been mentioned.
However, if the owner was unaware that the animal had it and it was exacerbated by the OP's wife jumping the horse, then what I see is that both the lessor and
the lessee are probably a lot less experienced than they want others to believe. Which means they point fingers at each other for the horse's condition.