It depends on where the tear is, what sort of fabric is it, how big, is it ragged, is it huge, is it small, and so much more.
Does the studio owner have any others of this leo that is the wrong size? Do they have one that could be cut so you could make a patch that is underneath the original garment? Velour leos are more forgiving than those with a metal sheen or shiny. Every little thing will show on those. You can tuck a larger one over in the crotch area and sew that shut so that it fits better and also take it up on the shoulders IF it is a tank style or something without sleeves.
If you can simply take some thread that matches and sew the edges back together that would be ideal. From the back side.
If it's in the crotch or hiney area perhaps other parts of the costume will cover it. If it's on the front it's going to be hard. She might not be able to be in the show.
But what I recommend over all over options is to talk to the studio owner right now, call her. Tell her what happened and ask her if she has a seamstress on hand. I make most of the recital costumes at my girl's studio and have done repairs.
IF you can order her another one then I suggest that you need to do that today, pay all the extra shipping cost for overnight or next day.
And the best advice I can give you is to not let her wear her performance leo to class. Only dress rehearsal for photos and for the performance. She should have a class leo that she can wear to just dance class.
Our studio owner had the kids use their class leo for one costume then they have another that is 100% for performance, part of a costume that is. She includes a new pair of tights in this costume cost and they aren't supposed to wear that pair until the day of the show. I always get a couple pairs of tights to have in the dance bag in case they tear while putting them on.