Get a good sports bra that matches the bottom of the top. easiest if it's black/white but in case anything does ride up, it won't be as noticeable if the bra blends right in. and some "stayput" that gymnasts use to keep leos from moving off the cheek into the crack. sprayed from skin to bra or skin to top if they touch.
and have her put it on at home and do some high v's, handsprings, etc. to see if things stay put. I was always super self conscious like what if my top AND my bra ride up? should I put a bandeau under the sports bra? (I am also a member of the IBTC so IDK why I was concerned)