Looking at the distances between islands, it's unlikely that microwave solutions will work for your friend. Sorry about that. The limits of commodity 802.11 hardware is probably on the order of 5-10 miles. Fancier licensed carrier grade gear can go a bit further, but generally in order to reach the higher ends of those distances, the sending and receiving antennas have to be at higher altitudes (
https://en.wikipedia.org/wiki/Fresnel_zone#Fresnel_zone_clearance), so either very very tall towers or placed on natural elevation peaks.
Physics can be a real PITA sometimes....