If age means nothing, why did she throw a fit when just mentioned the fact that her horse is herd bound? Its nothing to be ashamed of.
If she's been riding since a young age but has only been on one other pony till now, then I wouldn't be putting on the impression that she knows what she's doing. If you've only ridden/worked with one horse for a long time then switching to a new one will bring up some issues in your ability to handle horses. Everyone has been there.
She has been on different horses at the high school aswell but they are all trained the same, act the same etc. I totally agree with what you're saying though, switching horses after a long time of the same can be hard. Something I'm not looking forward to doing. :(