Supported touring is really just doing a series of day rides consecutively. So any bike that he's comfortable taking on a fairly long one-day ride should also be fine for a supported tour. There shouldn't be a need for a rack, but it's pretty easy to add one to just about any bike with just a bit of ingenuity, so I wouldn't use that as a criteria to choose a bike.

Now if he later wants to go on a self-contained tour then some thought should be given to getting a bike that will handle well with a substantial load - but it doesn't sound like that's the case here.