With a little adjustment it might work; with a few provisos at least.
As far as I know (not a girl), halter style tops are uncomfortable for very large chested girls: they don't distribute the load of the bosom as well and can stress the neck very quickly. It would be better for a smaller girl, but as long as the fabric is strong enough and provides enough coverage/support, I don't think it would spill outright.
I wouldn't want a girl to run a 5k in one (or jump very hard in it for that matter), but it should work for lounging at the beach and looking good.
