She should talk to her trainer. The kind of horse she wants will be expensive and fairly rare. She could also reach out with her trainer’s blessing to the higher level dressage barns regionally.
Is friend capable of riding at that level? Has she competed at that level? Or is she a lower level rider hoping to move up with a horse that’s better trained than she is? That doesn’t always work so well.
If she is indeed training at 3rd or 4th or higher with a competent coach who has connections in the discipline then the coach can take the lead shopping for a horse for a commission. At 10 per cent, the commission on a horse of this caliber will be worth the trainers while!!
If your friend is dabbling at first level and just wants to buy a made horse on the open market without being in a program there are not really dressage supermarkets where you just walk in and buy your dream horse. For one, the trial ride will likely be a disaster.