Obviously snorkeling fins with socks won't work most places in Bonaire other than hotel docks and such, unless she does the Tevas to the water thing and straps them someplace while snorkeling, maybe to an empty weight belt if she's not wearing some kind of vest. So this has gone on for pages but is there any idea if the person in question is willing to return what she got and get something else? If not there's little point. All you can do is tell her to bring sturdy waterproof sandals so she can do the swap thing, and she can sit out any sites that don't work for her.
While it seems more people are fans of heavy soled boots, I agree lighter soled tropical booties can be a viable option, even if they probably won't last as long. Being able to feel your way more and finesse it a bit can work well. This may be even more true for someone snorkeling rather than heading across the ironshore with heavy gear. Maybe a little less comfortable than heavier soles, but I tend to think this can work better and be safer than the sandal option, with less likelihood of tripping or trashing your toes.