Why not get her a bike she'll quickly outgrow? The XR100 may be the best bet for a real beginner. She'll be close to the ground, she can pick it up when it falls over, and it'll be easier to learn on.
Once she gets the basics down, she'll want something better, then she can step up to an XR200, or a KDX200.
Buy a used XR100 and let her have at it for a few months. When she's done with it you can sell it for the same price you bought it for!
Speaking from experience, a small, lightweight trainer bike is much more confidence inspiring than something you have to ask your parter to help you with after a crash.
BTW, when she's ready for the XR200, you might want to look for the 86 - 93 models. In 1994 they went to a shorter (slightly lower and more compact) chassis and cheaper suspension components. The earlier model is more fun to ride at her height. The new model feels cramped on long trailrides for a 5'5" person.