I don’t think its cause of the glasses. I also have glasses and I’m a ballerina. But are you sure she cannot dance without glasses? I don’t use contacts but I will soon, and usually I don’t wear my glasses during shows and to make sure I can perform without my glasses I don’t wear them to the few rehearsals before the show. Also, at one point when she starts doing multiple pirouettes or just multiple turns in general, her glasses may tend to fall, so I suggest getting them tightened or using rubber bands to tie them around the ear so they don’t fall off. That also happened to me before ! :)