Thanks everyone! Her incident with the round pen was about a year ago, and she has been out to pasture ever since. She was checked the first day I was out by a vet, and he checked for pain.
Fast forward 6 months to my first trail ride with her and as I'm warming her up I notice she's stiff to the right. Not a lot but.....it's there. When we got done with the ride I tried carrot stretches with her and found that she really had a hard time flexing right with her neck.
Over the next 6 months I had the Chiro out 3 times to adjust her and each time we'd get better and more flexion. In between visits I'd really work her on carrot stretches and lateral flexion. She's fine today and she never took an off step or bucked (her 'out' was in her atlas/poll/neck not her back) and it wasn't a HUGE off but she was off and all she did was throw a couple of little tantrums at the trainer's place.
So, if you still have bucking issues after a few months working with this mare, I'd really have the chiro out to look at her. Vets are not always in tune with the kind of thing that a good chiro can find.
You also said the owner pushed you into getting on the mare and got mad when you got off. If he tries that again, you need to establish who the trainer will be, you or him. What he wants at this point is only a small part of what is going on. While you need to be mindful of his pocket book and not take forever to either get this horse back to being a sound minded horse or to call it and say it isn't going to happen, you don't want to allow him to rush you.
Work with this mare for a few days and sit down and draw up a training plan with what you feel are achievable goals for the first month, the 2nd and so on and sit him down and give him the plan. Then each month touch on the plan and detail what goals you have reached and tell him about it, sooner if he is one to come around a lot. That way he can see with his own eyes where and what progress is being made. As you go along you can adjust the plan to reflect where the mare is, training & trust wise, and if she's going faster in a certain area be sure to emphasize that and also let him know if you hit a road block and define what you're doing to work through it.
With a horse like this, a lot of good communication will go a long way to keeping her owner happy. If he's a true horseman, and even if he's just learning to be a horseman, he'll be able to see you're not just taking his money for the hell of it.