If your friend is from the area he should be a little familiar with some of the dive/snorkel sites. If not I would suggest looking up several that you would like to do and hopefully the boat will have a GPS that will take you right to the spots. You can pick up reef maps at pretty much any place down here in the Keys.
Are you wanting to do more shallow spots so that you can dive while they snorkle? I would suggest a lot of spots on The Elbow, The City of Washington, Molasses Reef, French Reef, The Benwood, Key Largo Dry Rocks (Christ of the Abyss). All of those spots are 40ft and under. Also, since they are all located in the Park they are protected and have mooring balls to tie up to, NO anchoring. Usually, the currents are never strong on the shallow reefs unless we pick up some really crazy weather. The currents can sometimes pick up on some of the deeper dive spots like the Duane and the Bibb but not as much on the Spiegel. Key Largo is known for its great viz and warm water, usually in the mid to late 80s in June. The majority of the reefs are large coral heads laid out in a spur and groove reef system, "spurs" of coral and "groves" of sandy channels. Makes for easier navigating. All of the dive spots are full of life, the Benwood being known as a very fishy wreck. French Reef is know for having several "caves" which are actually some really nice swim throughs. Try checking out the Scuba Shack website, we have an intereactive map which will tell you a little bit about each dive site. Once you see where they are all located in relationship to each other you may be able to create a bit of a schedule. Fee free to contact with questions and have fun!