By your description there could be many causes. The most common no start I've run into is bad connections. The battery will be fully charged, but all you get is a click. Start by cleaning (or replacing if necessary) both battery terminals. Double check the connections at the solenoid, starter, alternator and block (negative) to assure there is no "green nasty" crap built up.
Before you try to start it again, follow brads70's advice and pull the plugs. Not knowing your friend, he may have mistaken what a "locked up" engine feels like. With the plugs, accessories, fan and radiator in the way he may not have had enough leverage to be able to turn against the compression and resistance. Pulling the plugs eliminates the compression and makes the engine easier to turn by hand. Then do as suggested and get a socket and breaker bar for the crank bolt and turn clockwise. It won't turn easy but should turn. Good luck!