Does your friend already possess some cycling skills (e.g. balance, stamina)? Depending upon his current physical condition, he might be totally fine as a stoker, or he might consider practicing spinning on a stationary bike for a while just to build up his legs and cardio.
In any case, I think that you'll both have a real blast riding a tandem bike together. There's nothing quite like it.
I think that his size draws more attention in public than anything else and if he's stoking on a tandem, people will see that he is being proactive in getting fit and should therefore be encouraging. Unfortunately there may be the occasional dickhead comment, but there's no way of avoiding jerks completely and they should not discourage him from doing what he wants to do.
Since speed and distance riding will likely not be a priority early on, a 26" wheeled stiff framed mountain tandem (as suggested by TandemGeek) would be a fine start.
Glad to hear that you're both interested in tandeming and if there are any other questions, we'll be happy to help encourage both of you!
.
Last edited by Stray8; 02-05-10 at 01:09 PM.