Based on my limited Keys experience, make sure it's a tropic line that can handle the heat. It may be best to buy the line down there and have a local shop set it up.
I've had tarpon on my 9 weight. It was a blast but I didn't come anywhere near landing them. If your buddy is using a guide and doesn't have any heavier rods of his own that he just has to use then I would rely on the guides equipment. That way you know it's going to be in perfect (or as near as possible) condition. My guide had all Scott rods and they were a pleasure to use.