If it is arthritis, it will show up on the x-ray. If it is soft tissue damage (like a tendon) the x-ray will be clean.
It if it arthritis, it could possibly be treated with injections.
As long as you have the money, it would certainly be worthwhile to dig deeper into the problem and actually find out what is going on. Do you have a GOOD lameness vet in your area?
It's not so much the weight of the rider that will make a horse non-rideable, but its the actions and movements the horse has to go through. If the horse is lame, it should not be ridden by a child or adult. Period.
∞•*˚ Βгįťţαňγ ˚*•∞
It is not enough to know how to ride; one must know how to fall.