Joe and Doug, thank you so much for the replies, definitely good info. I hadn't heard of the Architect's Digest before, I'll see if my local library has it.
Those diagrams and the article look like they'll help, too. I'm thinking about two possible approaches. First, I could add a few inches to the measurements in the "tall" diagram, but that just seems risky because it could throw off angles and ratios.
Second, and the approach I think I like better, is following the article which said the average height is 5'10" (or 70"). My friend is 7'2", or 86". That equates to a 23% increase over average. I think the math might come out better if I just say 25%.
So I'm thinking about finding a good plan, and just increasing all of the measurements by 25%. I may not be able to play around that much with board width, so I may have to do something like add some extra seat and/or back slats.
On a side note, I can find dozens of plans for these chairs. Any recommendations?