Ditto what Veronica said. Shelf-bras are pretty much the best you can find built into a jersey. If she just wants a camisole type top, a few companies like Sahalie (aka Early Winters) and TravelSmith have camisoles in 3 different lengths, some with underwire. Some have more cotton content than you'd want in a jerey, the length could leave her lower back exposed to the sun and there will be no pockets.
BTW, the "smashed" look is commonly refered to as the "uniboob." Now, that you are enlightened, bravo to you for encouraging and helping her ride!
Frends know gud humors when dey is hear it. ~ Da Crockydiles of ZZE.