Recently there have been several posts my a member seeking information for a reasonably priced and sturdy bicycle for his 275 lb body. After some thought, I have a solution, though it will be controversial to some.
I believe the best solution is a good quality ATB. Most road frames regardless of the tubing have recommended maximum rider weights of 200 lb, but good ATB tubesets do not have weight limits. While they are heavier, by virtue of their thicker walls, this is just what is needed to safely carry someone of this weight. Yes, I realize that that the frame designers have built in safety factors and that many of us have ridden long term on frames that were not rated for our weights, but for this rider, this seems to a big concern and I believe he is best served by an ATB tubeset.
The high rolling rersistance of knobby tires is easily overcome by going to high pressure, non-knobby tires such as Specialized Fat Boys.
The wide range gearing of an ATB is probably more appropriate for an unconditioned novice when trying to move his heavy body up a hill.
The wheels, provided they have box-section rims, should be be better capable of handling the load than a road rim.
The lack of hand positions are can supplemented with a pair of bar end extensions, to make climbing easier.
So, in the end, I suggest a good, used, medium level mountain bicycle. Something with box section, aluminun rims and a good ATB tubeset (such as Columbus Crommor OR, Reynolds 501 ATB, Ishiwata MTB, Tange MTB or the equivalent). Add a set of high pressure, non-knobby tires and bar end extensions to make the road riding more practical. A touring bicycle with an ATB tubeset would serve just as well, but these are a little harder to find than an ATB.