As a ballet mom first, simply because I have been a ballet mom for 9yrs, gym mom for 3.... No, a gymnast won't achieve the fluid, classical ballet lines from 1hr class/week... BUT any ballet/dance will definitely be beneficial! Through the years at the studio I have seen a few gymnasts who took ballet simply to help their gymnastics and it did help their lines, fluidity and leaps. None of them kept going after about L7, I'm guessing for time constraint but I wasn't into the gym world enough to think about it much more t the time. Because your dd is a serious gymnast first, her body won't be changed by that amount of ballet. I think it's great that she wants to take a serious ballet class and it will only help! The opposite is almost never a good thing (a ballet student wanting to take gym on the side for strength and flexibility) but some, ANY ballet is almost always beneficial to gymnastics. Have fun with it, I wish my dd would want to take class, she could technically do it on the one day she has off from the gym because we're at the studio anyways but it's her mandatory homework night.
I'm so jealous of the people who have mandatory dance....
I think the ideal would be to have a dance/ballet teacher come to the gym on a regular basis because if you are simply taking class in a serious ballet studio, you aren't going to get as far into ballet training as you need to really see that "dancer training" unless you take multiple classes per week, which gymnasts don't have time for. But if your gym has someone to come in and work specifically with the gymnasts and what they need.... That would be so wonderful!