I am a 5 foot 10 inches retired back-spot with a D cup and totally understand your problem. The last team I was on had a weird top that was a tube top with straps over each boob for support and basically crossed in the back. The tops elastic barely laid over my bra by maybe a half inch. My top also was extremely tight and small (I had 1/2 inch of cleavage pooping out) and was a struggle to not only fit over my chest but stay as well. What I did was wear a cheap thin spandex sports bra that maybe a cup or 2 small and then wear a actually clear strap bra, not a sports bra with clear straps but an actually bra. It worked, everything was nice and snugged and when I would tumble, lift my arms, or stunted, only the underwire would pop out. Which is better then a malfunction. I would say try that or use tape to basically lift her chest as high as they will go. Or, use double sided tape and tape the top elastic to her skin.
Hope this helps
Sincerely,
A fellow Tall Girl