Been to Guanaja but not to Posada del Sol.
If your friends are expecting anything like AC, well… you better tell them to find another place. For the most part they have resorts/all-inclusive ones. Travel is limited by boats/water taxis or wheelbarrows, what are you friend’s real reasons for going there, diving, fishing, or just relaxing, a little of each? I like the diving, mainland US divers have not trashed it out and the cruise boats aren’t there.
Forgot to mention that January is in the rainy season, could be sunny or rainy, and don't forget about the bad 'northerns'