What, other than the fact that she's green, makes the "friend" think she's too much for you? She seems to be above average for a young mare in training. I was impressed with the videos; she is a beautiful mare, moves well and seems to be very sane, and a good worker. If you're a beginner rider, I can see that it would be better if you weren't 'green on green.' Howevr, I have to say that my first thought was that anyone who wanted to buy her from you before she's finished with her training, supposedly in your best interest, is either scared to death for you, or is impressed by Misty and wants her for themselves and is trying to scare you into selling her cheap. Do I sound paranoid?
FYI - In the midwest (Missouri-Kansas) where I live, the going price for well broke grade horses is usually far less than the cost of the training. It's not unusual to see prices of $800-1000 for registered, dead broke horses. It isn't hard to find young ranch-trained horses for around $1500.