Well in a sense it may be fair to say that any good riding and training that the horse gets will have an influence on him just as the opposite would be true. If it's possible to teach a horse to do something wrong then it's possible to teach him to do something right. What you would do would have some amount of carry-over to the next person, to a degree. However it's also a general truth that the horse performs to the level of the person riding him right then.
The better question to be asking perhaps (as others have pointed out) is whether the horse is being over-used. To say that he has 7 lessees doesn't tell the full story - I've heard of this kind of part-lease agreement before. Of greater importance perhaps is in how well it's all being managed. Is the horse being over-used? There are ways to determine this that are worth learning about if the goal is to have this thing stay consistently sound so that it can perform the required duties. How often and how hard is he being rode? What's the level of care like? How well does the tack fit? The more you use a horse, the more important these things become. These animals do actually have a fair amount of strength and endurance that you can put to your use, and in fact it's not a bad thing when you compare it against all these horses that just stand around in a stall their whole lives but the owner's got to take responsibility for them. If the situation is that a bunch of yahoos are yeehawing around and jacking the horse up all the time then that's not good. But that doesn't mean that these things can't be done well. It's just that if you have that happening, then the owner isn't doing a good job of managing their horse. But if that's the case, I wouldn't worry about it. They'll find this out for themselves when the poor thing breaks down. If it were me though I'd just try and learn from their mistakes and move on. Use their experience to make you better. That's one way you can try to do right by a horse that you may feel sorry for but can't really do much to help - use their experience to help the ones that you can help.
Last edited by Ian McDonald; 09-27-2013 at 02:23 PM.