There is a reason why there is a warranty on vests . . .
Many vests are worn for AT LEAST five years which means you buy panels that have been subjected to extremes in temperatures, hydrolosis, flexing, folding , poor storage, possible separation between bonded layers and so on. Plus, when they get replaced, they are usually pretty frausty (ewwww). Some have been known to wear the vests against bare skin. So no matter if someone changes the cover, you have "da funk."
While the adage of "any vest is better than none," does your friend who owns a business want to use used vests to protect his officers? A failure of an expired vest will mean a huge lawsuit and the end of your friend's business. Regrettably, the initial cost is why many security companies do not issue vests.
I applaud your friend for thinking of them but please reinforce that used vests are not the deal that they may seem on face value.