Ok, so what I would do if I were you and chose to lease this horse is learn to ride western pleasure. What will it hurt you? Nothing. It will make you a better rider and at some point help him if he gets too far out of his comfort zone under english. I say this because with my mare, I rode differently for each discipline. Different bits and all. That way she was not confused on what we were doing. I also say that because this horse (I would guess) is probably a slow mover. English riders prefer a more forward moving horse, am I wrong? I ride a little stud out at the rescue ranch I volunteer at now. I've put a pretty good start on him, and it's more geared toward reining as that's his natural talent. Now, there are other volunteers at the ranch too who grew up english and that's all they know. I was sick for about 2 weeks and he was ridden by them. Now, he moves out a lot more then before and has a harder face. Not saying this is an english problem, but he's obviously a little confused by the different riding styles. No big deal to me, just means more work. The reason I say that is because you mentioned that the owner still plans on showing her horse this summer. IMHO, you should try to keep him rounded in both disciplines. It will make him a better horse, and you a better rider. So, go learn some western that way you can effectively communicate to him, keep him mentally versatile and the owner can still enjoy the hard work she's done on him when she wants to. Also, I am NOT bashing english!! Just saying that confusion can come when mixing the two too fast and in an unclear manor. I am NOT saying you will ruin this horse, but you need to learn from him before you can teach him.