Need help installing OS X El Capitan on Mac

A friend gave me his Mac. He deleted/removed (or whatever you call it) the OS and all programs and data before giving me the computer. He told me how to reinstall the OS but I'm not having any luck getting the OS installed. When I start the computer and sign into my Wi-Fi network, click on Reinstall OS X in OS X Utilities and hit continue, sign into the App Store, I get the message "This item is temporarily unavailable. Try again later." From what I've been able to gather the computer will only allow the original owner to reinstall the OS. Does anyone know how I can install operating system. Do I need to have the previous owner do something online?