I own one, and love it. It's a double-action auto. That means the first trigger pull is longer and heavier for safety reasons, subsequent trigger pulls are shorter and lighter. Decock only means the levers on the slide lower the hammer and then spring back, they cannot place the gun "on safe". The safety is provided by the long first-shot pull.
When you load the weapon you should insert the magazine, cycle the slide by hand (finger OUTSIDE trigger guard!) to chamber a round, and then depress the decock lever. This lowers the hammer for the DA first shot. You should also decock after firing a string.
Some folks like this system (including me), others have trouble with the two different trigger pulls or feel better with a manual safety. Hope your buddy enjoys the Ruger; it's a best buy for reliability and long service. Since the manual should have explained how the decock-only works, I'm guessing your pal bought it used and did not get the book. If you go to http://ruger-firearms.com you will find an address to write to; Ruger will send you a manual, free, for any of their products.
