If your friend is comfortable, can she ask him to lope while riding him bareback? It might help to rule out if it IS a tack issue, if he lopes fine while riding bareback.
Keep in mind, if his saddle IS hurting him, he may still buck initially the first couple times because he is anticipating it to hurt. It takes them a couple times to go "Oh, hey! I guess this doesn't hurt. I guess I don't have to buck in protest."
But of course, if your friend isn't comfortable riding him bareback with a possible buck, then don't do it. We don't want anyone getting hurt!
If someone else rides the horse besides your friend, do he still buck? If your friend is tensing up and causing the horse to tense, that just feeds off of it.
Since you have had the chiro, dentist, and massage therapist check him, the next potential step would be to have a lameness specialist vet check him. I use this entire TEAM approach to care for my horses, because one may spot something the other doesn't.
Is there a saddle fitter in your area? Or somone knowledgable about saddle fit? It would be worth checking out since the horse will lunge both ways without bucking but will buck with a rider ONLY to one direction.
If it's not the saddle, and if the vet says he's fine, THEN you can progress to behavior attitude changing.
My horse Red will test me from time to time. When I first got him, he HATED loping circles. HATED it. ANd he would protest by bucking. I suspect that got him out of work with his previous owners (who told me he never bucked ..... but then again, they never made him actually work!!). But it did NOT get him out of work with me. I kicked him harder, made him continue going in a circle, yelled at him, and when he was finally loping nicely, THEN I allowed him to stop and praise him.
But the main thing is that when they start bucking, you have got to ride through it and do not let them stop loping. If they see they don't get to stop working from a temper tantrum, then they will quit pushing your buttons.
∞•*˚ Βгįťţαňγ ˚*•∞
It is not enough to know how to ride; one must know how to fall.