EJ Manuel is exactly the kind of player Dallas has shown interest in signing in the past. He's a former first-round draft pick who hasn't quite lived up to expectations. He would be a reclamation project in every sense of the word.
Manuel has all of the intangibles you look for in a QB, but for some reason hasn't been able to put things together in the NFL. Of course, that could be because of the situation he has been in with the Buffalo Bills. EJ hasn't had many good weapons at his disposal during his time in Buffalo, but that's not the only reason he hasn't found success yet.
For one reason or another, Manuel's mechanics have regressed since leaving Florida State to enter the NFL. It could be all a matter of confidence, but he doesn't trust what he sees on the field and often makes bad throws because of that.
If the Dallas Cowboys were to take a chance on him in free agency, it would likely mean they believe that a change in scenery/coaching could bring out the talent which made him a first-round pick to begin with.
He has the size, strength, and mobility to be a good backup QB option behind Prescott, but the Cowboys would likely have to start from the bottom to build his confidence. I don't know if they're looking for a reclamation project, but it could pay off in a big way if they are successful. Since Manuel would cost next to nothing, it's not the worst idea I've heard.