That is "treatable" with lots and lots of transitions over lots and lots of hours. If you're not being paid for your time I would say that gets filed under "not my[your] problem". A quick fix would be a stronger bit but if you go stronger and the owner has hard hands, you will continue to go harder and harder to get any kind of reaction out of her and that is an endless loosing battle. The only other alternative I could suggest would be a hackmore and that's not English. An elevator bit will give you leverage without a particularly strong mouth piece but even that will not fix the problem, mask it, but not fix it.

A strong tree is my best assistant- release and tension are perfect, works in the sun and cold without complaining, does not need a lunch break and I don't have to pay it.
www.thompsonpaintranch.com
trainthehorse.blogspot.com