With my horse who is 15, I have to jump him at least 5 times a week. I used to do lots of dressage with him, but he wasn't the best at it and we would both get frustrated. When I moved barns the new trainer suggested jumping more because he loved it and mixing in dressage between the jumps if that makes sense. The point is that as long as the horse is sound, and wants to jump he should be ok. He does need breaks though, and maybe only ride him 5-6 times a week?? It depends on the horse though, and a vet would be the best person to ask. I would take it easy just doing a few jumps and not too high untill you have an answer. Also try talking to his owner about it and see what she says.