So, it might peripheral vertigo or it might be motion sickness. There's two types of vertigo, central, caused by problem in the brainstem, and peripheral caused by a problem in the inner ear.
Your inner ear sends messages to your brain about how your head is oriented. You have 3 semicircular tubes in your inner ear, one for each plane. Inside each tube are small pieces of sand (not really sand, but that texture). The sand moves with gravity and little sensors in the tubes sense where the sand is, and send a message to the brain that allows it to triangulate what your heads orientation is. The system can fault in two general ways. The most common is some inflammation that can either cause the sensors to trigger when they shouldn't (i.e. sand isn't resting on them), or not trigger when they should. This is self limited. Less commonly the sand bits can get stuck or dislocated. This is usually a result of violent head movements (most commonly amusement park rides). Your brain eventually re-calibrates itself, but this can take several days and the vertigo can be very severe. Both cases improve with medications (like antivert or benzodiazepenes. These medications work by attenuating the signal the inner ear sends to the brain (which is why they can effect balance negatively when taking for other reasons).
Motion sickness is different. The brain uses three inputs to determine position and orientation, messages from your inner ear, small receptors in our joints/hands/feet that sense position, and vision. When the brain gets conflicting information it can think you are moving even when you aren't, or think you're stationary when you're not. In cars/planes/boats it occurs because, if you are not looking outside the vehicle at stable terrain, your ear/joint sensors sense subtle movement, but your eyes see stability (since the car/plane/boat is stable from your point of view). That's why reading/watching a screen can make motion sickness worse, and looking out the window can make it better. This can occur in skiing in whiteout conditions (can't see a stable horizon), and sometimes with skiers that constantly look down at their feet. The same medicines that work for vertigo also work for this, by the same mechanism, you need to be cautious as they are working by inhibiting your body's natural balance mechanisms. First thing I'd recommend is to make sure she is looking down the hill when skiing. If she already is, then it might be worth trying a motion sickness medication (with the caveat that she should ski easy stuff when she first takes it to see how it effects her).
