Find your local Trek dealer.
Make sure the shop actually measures your friend for a frame (they'll measure his inseam, and his reach and stuff like that). Then (with Trek and a few other brands) they can recommend a bike with a certain size frame and frame geometry that will fit him best and allow him to ride comfortably.
Regardless of what brand you get, you want to make sure it's got all Shimano shifters, sprockets, deraileurs and all that. Front shocks are nice, don't bother with rear shocks or the crazy bikes with shocks all around. Unless you are really doing mountain biking you don't need anything more than the front.
Quick release wheels and seat are a big help, and disc brakes are worth the added initial expense if anything just because you don't have to adjust them later.
For a men's bike of a decent brand, you're looking around 400-450 for the middle-lower end. For your friend his height might limit him a little because most bikes are designed around 6 feet.
Whatever he buys, make sure he can get toe clips (they'll cost a few bucks more -- I like the ones with metal pedals and plastic parts), bar ends (these are the upright add-ons to the handle bars and are generally a cheap option), and I'd recommend getting a cheaper Cat-eye (little bike computer -- seems extra but it really helps you keep pace and meet your goals). Those are all options, but in this economy you should be able to negotiate some sort of deal on them if you buy it all with the bike.
Make sure he can almost lock out his knee on the downstroke when pedaling while not feeling like he is straining his neck (it's a combo of frame size/geometry, then seat height and handlebar height).