Why did he call Netgear ?
They have nothing to do with changing the internal or external IP address.
Did you change the ip address of the router before you sold it to him ? If so then he will need to type this ip into his web browser, then he can change it to what he wants. He will also have to key in the user name and password of the router. The default user name and password for netgear routers is usually
user = admin
pass = password
The default ip address is normally 192.168.0.1 or 192.168.1.1