Impossible To Ride Buck *Help!*

A friend of mine came to me today and asked if I knew how to fix her horses bucking problem. So I told her what works for me when my horse bucks, kick him forward and pull his head up, and she told me that her doesn't like to be kicked or her head jerked on, and that she has a massive buck. She's a sixteen year old Percheron/Thoroughbred, and she wont canter on the lunge either. She has always bucked going into the transition, hasn't had any known past injuries, they've had the vet come check her, currently not ridden with a bit, other people have tried to ride it out. They've had her since 1997, when a friend had her rescued from slaughter.
I've come up blank, I've never had a problem like this. Surely there's some way to fix it, unless there is a physical default? Does anyone have any ideas?

If she has bucked on the transition for ever, time to give up and make her a walk trot horse!

If it's new then time to check saddle fit, etc.

I don't think the horse is SUPPOSED to like it when you do that.... isn't that the whole point? Your friend doesn't like to be bucked off either but the horse doesn't care.

I would have the vet check her, her saddle fitted and rule out any physical pain or irritations first.

Does the horse canter on her own in the pasture?

I know how to fix her bucking problem -- you just would not like my solution.

This is one very spoiled horse that has wasted feed and money for at least 14 years. Maybe, just maybe her first owners had the right idea. I just cannot see a single reason to risk life and limb on a old spoiled horse that has been spoiled this long.

Yes she can, but it is my understanding that she still bucks

I rather doubt the story of a horse so sour she even bucks out against herself....

Is she sour? She's not my horse, I'm just reciting what I've been told. She seems like a fairly quite horse, walks and trots with no problems. I thought it sounded like a physical default, but my friend says she's been checked by her vet, so...

If she can canter on her own, that decreases the likelihood of it being a physical problem. But if she's bucking at the canter on her own....I'd say that's unrelated to the bucking under saddle. Bucking at liberty is a "feel good" thing. Without seeing the horse myself, can't say whether I'd be willing to rule out lameness or not. Can you get a video?