Her brush needs to be consistent. Beyond that, you are approaching the problem wrong. She needs to work on her learning how to move the ball inside/outside/down/up.
She is throwing inside--which is a good pitch. Pitchers throw that pitch all the time. What she doesn't know how to do is throw the ball outside. She doesn't know how to throw the ball inside/outside/up/down on command.
How much does she practice? Is she putting in the time to master control?
The only person who can teach her is herself. She has to learn to control her release in terms of microseconds.
Let me clarify, her accuracy is actually quite good but when she misses, most of the time it's inside. We practice frequently and constantly work the 4 quadrants. She prides herself on her accuracy, which is why she gets frustrated when she misses inside.