Originally Posted by
Often1
The answer is that it is not allowed, is actually illegal and, at a minimum will subject you to a world of hurt if you are ever caught. For what it's worth, TSA does random secondaries at the gate, often checking ID's just to root this out. At that point, you're standing at the gate with a useless ticket and either purchasing a seat at walk-up fares or slinking away.
Except you don't need ID to fly - if you know the specific personal details of the person whose ticket you are using, you can just show up, claim you lost your ID and answer their "special questions" with answers supplied by the 'friend' - that will take care of the TSA and the airline.