Thank you for the response. I am still not able to determine the exact model MBA that you have. Ask your friend if he can provide you with that information.
If your MBA is a late 2010 model or earlier, you will need the original install media to install a working OSX. The seller should provide you with that. If not, you will have to contact Apple Customer Service and get a duplicate. There will be a charge.
If your MBA is a 2011 model or later, boot the MBA with the OPTION+COMMAND+R keys. If successful, this should result in a display with a revolving globe. This will connect your MBA to the Apple servers and allow you to install the original OSX that came with it. Follow the installation directions and you will need a solid Internet connection. If this does not work, it suggests that you have a 2010 model or earlier.
Ciao.