try searching the forum for "big guy, kayak" or something along those lines and there should be plenty of reviews. if not a good place to start would be a nucanoe frontier, or jackson big rig. they won't be speed demons, but they're known for their stability. there's a few demo days coming up so i'd highly suggest going to one and talk to the people there. they should steer you to the right type of yaks.
Is he going to fish lakes or bays? What's the budget? How is he going to transport/store the kayak (shorter kayaks are easier to store/transport)? Does your friend want a kayak which has a more stable hull design or one which is more efficient?
Given his weight he needs to start looking at kayaks with a weight capacity of 375 lbs and higher (which is a lot).
At the least, ask your friend to try on for size. At 6'7" he may not be able to fit due to ability to extend his legs. Pay attention to comments by Neumie, he used to work in a kayak shop. As far as calm waters go, some day your friend might get caught in rough conditions.