I hope I won't be beat up for saying it, but there ARE exceptions when people with limited experience take in a youngster and successfully bring it up themselves. With that being said it's something not very common, and something I personally would NEVER recommend.
With your friend you may try to bring up your concerns in a very nice and friendly manner, because there is a good chance she may be offended. If she still wants to get a "tough" horse, try to persuade her to work very closely with the trainer for the sake of her own (AND horse's) safety.
"Life isn't about waiting for the storm to pass: it's about learning to dance in the rain..."
"When we are no longer able to change a situation - we are challenged to change ourselves."
"How people treat you is their karma; how you react is yours."