My horse has the same issue except to the left which is his bad way. And with my horse its half an attitude issue and half him being unbalanced. My horse was vet checked for lameness, has his teeth checked and floated annually, and I have had my tack checked for fit by someone whos far more knowledgeable in that area then me and he was loping fine and then started bucking when I asked him to pick it up.
With Cody (my gelding) we decided to back track. I worked on teaching him to bend and be balanced to the left and right at a walk, dog, trot and did LOTS of loping on the lung to the left (his bad way) to get his balance up.
We had to stop training for approx 3 months due to medical issues. But he's cleared up and I spent a few weeks or so getting him back into work. Yesterday I lunged him with his saddle on and had him going really well and listening to me both directions, and then when he was cooled out I got on jogged and randomly asked him to lope. And he picked it up no problem with no bucks on the right lead. Could have been a fluke but I was happy. Maybe she could try lunging him and doing ground work before getting on him to make him use his body and listen before hand?
What I also try when I horse has behaviors is a) disengage hip/ tight circle b) kick him and keep him going (which might lead to another buck depending on the horse), c) slam on the breaks, back up, and ask into a lope again.(which may not be ideal depending on your horse) whatever you do don't STOP because you stop he gets rewarded. Trainer or not I wouldn't suggest getting off, but that's just me.
I don't know hope I helped a little. Feel free to PM me, maybe through discussion I can help out more. I just can't think of anything off hand.haha