If you don't feel you can approach him directly without consulting your barn authorities, tell your barn manager, owner, trainer, staff, and vet (if they are around the barn a lot, at mine she's there every other day and tells us stuff about psychology and physiology, she's part of the staff) that you think someone needs to step in and will yourself if they don't mind. Don't say 'Bob's abusing his horse!' and be a drama queen. Just be like, 'I strongly disagree with the training methods Bob is using and, if your opinion is the same, I think we are obliged to make it clear that what he is doing is abuse and unacceptable. The next time he is here I think we should talk to him and supervise him.' Of course, being open with 'Bob' is better but I understand the need for 'backup'.
Also, I think you should really have a talk with your mother. I doubt she really thinks animal abuse is acceptable. Maybe she didn't understand what he was doing?
~Claire, the frog in the desert