I would kill for the barns I was out to treat my horses to peppermints! I don't understand why its an issue? But no they arent a choking hazard my boys and all my other horses at the barns I've managed got them to and I never had a problem. If you don't like it then just tell her, just say I would prefer it if you did not give them any treats they are on a strict no treat diet or something to that affect
"The horse you get off is not the same as the horse you got on; it is your job as a rider to ensure that as often as possible the change is for the better."