Welcome to FlyerTalk!
No, one does not have to pay for EconomyPlus.
If all the Economy seats are taken, he can go on a list of folks who need seats. After no-shows are processed, he would have gotten a seat. If only EconomyPlus remained, he would have been given that for free.
It sounds like he may have misunderstood what happened, or the agent was a bit too aggressive in selling E+. Usually they'll say that the only thing they can confirm are E+ seats for $, or he can wait until the gate to get his seat.