I love Bonaire, but you don't get much sandy beach unless you stay at Harbor Village or Eden Beach Resort (I think there's another exception or two, but Bonaire is not a sandy beach destination), and the glory of Bonaire is the dive freedom (when, where, how much & how long you wish) made possible by shore diving. Which is work. I also recommend people headed to Bonaire the 1st time join a group to help them learn their way around, since driving along the west coast in a truck is needful to maximize the experience.
Curacao is larger, said to have sandy beaches, and offers shore diving though the reports others post don't seem to 'shore-only' focused. Haven't been diving there, but it sounds pretty good.
I've been reading up on Grand Cayman a bit; from what I'm told, overall a pricy destination, airfare & car rental tend to be surprisingly affordable to factor those into your total trip costs when comparing to competing destinations, and a couple of reputable 'near beach' options that caught my eye were Sunshine Suites and Comfort Inn (the latter's nearby beach got trashed in a recent hurricane, I'm told; I don't know how fast it'll recover). Some deals at Turtle Nest Inn, car included, sounded great, if you're willing to get away from Seven Mile Beach and do some driving.
St. Croix could provide what you want. I imagine St. Thomas could, too. You might read some reports on Ambergris Caye out of Belize. Roatan is a popular dive destinations; I don't know off-hand who has & doesn't have much sandy beach.
How important is it for her to get to some population center for colorful Caribbean buildings & shopping? Someone who stays on the reservation vs. someone who likes to roam can change recommendations drastically.
Richard.