Well, having one address doesn't mean it isn't safe, it just means that if anything happens to that one address that it would affect his entire wallet. If he knows what he is doing, one address will be all he'll need. People just suggest multiple addresses for the partially increased anonymity and the ability to divide your funds among the wallets to prevent you from losing all of your funds if something happens. You can have 5 addresses on one hard drive and if the hard drive gets corrupted or is damaged, that's the end of the entire set of funds.