Your Computer -> Router (with it's IP) -> Friends Router -> His Computer In the case you use Skype since it's peer2peer
With a VPN this will happen
Your Computer -> Router -> To Server/Router of the VPN Service -> Friends Router -> His Computer
So as you see, your friend will now receive info not directly from you but from a server that you hired to send your information to him. So if they search for an IP they will get the IP of the VPN Server and not of your Router.
I know it's a little more technical than this geeks, but it suffices the answer for him