Mr. Punch,
I have had this (type of) student. I put her through our standard program. The bouncing around during two person exercises will be the experience from which she will draw from to understand how she must use her advantages. Make sure your other students are not malicious, but purposeful with her. Don't worry, if she is determined she will find the way to make the Kung Fu work for her body type. As a beginner, she must be patient and you nor your students must not feel obligated to make special arrangements for her. I agree it would be a good idea that she plays exercises on a horse or from some seated position where she can focus on the hand positions first.
Remember that the Kung Fu becomes dynamic over time. In a static case, yes, she might be very vulnerable to force and power. But over time, as you have mentioned angling and hips (which done properly needs time to develop), she will gain some ability to handle this. Like the Yin and Yang, we must experience the opposite action to understand the action (cannot learn to relax without understanding tension).
Good luck....
Moy Yat Kung Fu - Martial Intelligence