If the coaches daughter is young, she's probably at the life stage where she has no clue. She's invincible, doesn't understand why other people "just don't get things", and because she probably grew up around horses and had the benefit of riding every day from a young age and learning rapidly, she doesn't understand why others don't instantly pick up on things as well.
Aches and pains that can slow down us more mature riders? She has no idea there either.
However, none of that is an excuse for ditching out in the middle of a lesson. That's just ignorant. Her mother really needs to call her out for that and realize that she's doing their business a huge disservice treating paying customers like that. Even when things are going well, with that sort of attitude I doubt that even a "successful" lesson is a gratifying one for most people.