A cracked guide rod is usually caused by user error in the disassembly process. No offense, not saying that's what happened in his case. Being it was used, who knows what may have happened to it. They occasionally do a come-a-part after 20k rounds though, so I hear.
The steel guide rods claim that they take "torque" out of the pistol, which in my opinion tightens the gun up. The beauty of the Glock is that it is loose enough to function better than a tight tolerance gun. I've also heard that the steel guide rods improve accuracy, but for the most part, Glocks are more accurate than their users to begin with.
If he want's a steel one, go for it, but for me, it's factory same-same plastic ones. Let us know if there are any malfunctions after the change.