As a barn owner I gotta pipe in here. I have an excellent farrier who charges a decent rate for trims and will do shoes if you want them. Nobody here wants them, so our barn gets done very quickly. I pay exactly the same as the boarders and I wouldn't consider doing anything else. I find that absolutely unethical and deplorable. I get a great rate because the farrier gets to do every horse on the place, not because I negotiated a kick back. If he wasn't so good and the boarders wanted to use another farrier, I'd be fine with it, but I'd be having a talk with him about their concerns. If he couldn't or wouldn't shape up, I'd use THEIR farrier if he was better.
I appreciate that your parents taught you to respect your elders but when it comes to taking abuse from an elder, they should know better too. I would let things go this time but if she ever comes back on you for the new farrier then I would very firmly and politely tell her that her horses are her problem and your horse is your business and that negotiating a price break with the new farrier is also her problem. If she could offer him the whole barn, he might indeed be willing to cut a price break for EVERYONE in the barn. My farrier would charge $50 if he had to come out and trim only one horse, but because he does 40 when he comes out, he charges $35 to EVERYONE. That's a pretty good wage for 1 day's work. And if he did shoes, it would be around $100 for all 4 feet with just basic shoeing.