I am certainly hoping for fruiting to be possible in the Florida Keys. I see no strong reason why not. We do not have freezes, and certainly many of the other locations in which mangosteen successfully grow are also subject to severe storms. Sure, our soil is terrible, but it's not that difficult to use compost and/or purchased soil. My mangosteen has been alive and well so far, and I have had it for about a year and a half. I have not found the plant itself to be nearly as finicky as many have indicated, but of course I am just growing it right now, not attempting to fruit. It is far too young for fruiting. It had a nice push of leaves last year, and now hasn't done anything for a year - it would be easy to confuse it with a plastic plant for all the little it does. But since it's been nearly a full year since the last growth, I'm hoping for another push of leaves sometime in the nest few months.
gpuccio, could you please post pictures of your neighbors' trees so we could see the conditions, and just to see what they look like? What your neighbor has is very rare (at least according to documentation, although there may be many more FL mangosteen owners who just don't happen to belong to any online or local gardening groups for people to know what they have). It would be nice to see more evidence that what many believe should be possible, really is.