If he wants to use cellular data plan as internet connection for this computer. He can do so using a tethering app, I've used Foxfi in the past. However this is a very BAD idea. For two reasons #1 it will burn through your data so fast its not even funny and you will get overage charges. #2 once your data runs high your carrier is going to start poking around your data usage and if they find out you are tethering they are not going to be happy. Although you can avoid them ever finding out, just by using a VPN for the tethered connection.
However if you want to turn your desktop into a wireless hotspot so your phone can use that connection. Your simplest is to just get a cheap router, plug your internet cable in there and than another from there to desktop. However if you MUST have a method of using the wifi adapter you mentioned you can probably find a piece of software that turns your adapter into wireless hotspot. However just a fair heads up, your adapter will not work as an adapter and hotspot at the same time. So to accomplish this you will need a wired connection to you desktop.