I listened to Donna give an interview at this year's national championships where she talked about when she started riding with her current coach. She'd already trained herself to survive without the ventilator for 45 min or an hour or whatever it was she needed to ride (my understanding is that she gets off her horse and immediately back onto her ventilator), but she needed to slouch in the saddle to be able to breath. The coach said she needed to sit up straight, and she explained that she couldn't do that and keep breathing. His response was something along the lines of "I can tell you how to ride properly, but you'll have to figure out how to make your body do it."
Apparently they understood each other. Look at pictures of her now, and how beautifully straight she is in the saddle.