I'm afraid the only solution is to reset the router using the button provided (usually on the panel along with the network sockets.
The User Name is usually admin and on most routers cannot be changed. Most likely the Password has been changed. The default is sometimes admin and sometime password. Check the manual for your router once you have pressed reset.
Be aware that reset will change other settings to defaults, so best bet is probably to contact your friend first and see if he recalls what he changed the password to.