The most common risk factors for this condition are trauma, steroids, tobacco use, and chronic alcohol use. A careful history may help bring to light possible causes in your daughter's case.
She may have injured her wrist or hand (even years ago) but doesn't recall the incident. Has she ever been prescribed steroids for an inflammatory or other health condition such as asthma or arthritis? Usually a history of steroid use can be traced back to within a few years of the AVN starting.
There may be lifestyle behaviors such as smoking and drinking that you (or her doctor) are not aware of that can contribute to this problem. These two risk factors are independent of each other. In other words, a person doesn't have to smoke AND drink to be at risk for AVN. Either behavior alone increases the risk of AVN.