Rupert, as some of you know, has breathing problems. He breaths really hard, but that's really all I know of the issue, as he isn't my horse. It makes it harder for him to breath but doesn't seem to really impair him in any way.. He's eager to go and usaully perfect. I've started jumping higher recently and today we did a few crossrails and one verticle with a break in between each one. This wasn't too strenous on him, but is it whistful thinking to assume I can keep riding him as I'm starting to jump more?
Have you talked to the owner to try and get a vet to take a look at him? Is it worse in the summer? It could be allergy related. Or if it is consitent it could be something like heaves(COPD). I think a vet should definitely look at your horse before you continue to do strenuous work. Good luck!