In that case, the way to do it is to call the Verizon sales dept. and negociate with the sales rep. a credit for the ETF that he's paying to Sprint (if they don't offer it the first time he should hang up and try again until he gets somebody who's willing to do it), it's called "discretionary sales credit".
Open up the line on the spot. Once he gets the phone, port the number to Verizon, and the Sprint line will be automatically closed. Voila.
DO NOT SAY A WORD TO SPRINT ALL ALONG UNTIL THE LINE IS PORTED OVER TO VERIZON.