Are you talking about a piece of equipment that holds a bike still when you say trainer? If not, the others are right, it is too small and the paths and roads are too busy, mostly with kids, she'd hurt someone, and won't get enough training. But if you have something to mount the bike on, or plan trips inland, it's worth you looking. Biking is very popular on the mainland so you should be able to find something. Try Bruce's Cycling club on [email protected]
or (501) 223 7979 or Hop Sing on (501)227 2089. Both are in Belize City.