I haven’t played the AX66 myself yet and I really don’t like giving any advice based on paper specs alone. However, considering the situation you’ve described, l would tend towards the more flexible option. Sounds to me as if she needs as much help from the shaft as possible. If it all feels too wobbly in the end, then you can always increase string tension a bit to compensate. Another point to consider - have you asked her which one she prefers in terms of looks? For many players, it’s very important to play a pretty racket to make them feel confident about it.