We're meeting friends on their sailboat, which will be on a mooring in the harbor. Any recommendations for transportation? Will they be able to bring their dinghy right up by the airport?

You will need some wheels to connect with them. What do they suggest, or are they there yet? If you will be staying on the boat I suggest either they meet you at the airport or you contact them somehow, There are a few shoreside places in town where they might be landing daily. A taxi will do if they don't meet you.

if you had wheeled luggage you can easily walk to the Port Bonaire condos, which have a dock. I don't know what their policy is of having people tie up there.