You could do some research up front, so you have selected some good models she can choose from. A great way to do this is do some online window shopping for the type of guitar (acoustic, electric, 3/4 size, etc); most places also include customer reviews.
Try to buy from an actual musical instrument store where they can do a set up for you. Chances are, the less expensive the guitar, the more it needs one, but either way it is good to have done and they should include it as a service for no charge.
...so ever since then, I always hang on to the buckle.