At last how to decrypt that?

I have a router Zyxel 660H and bought it bf 3days.The same day i visited a friend which changed router's default password from admin to --> john and then placed username and password of his adsl connection.When i tried from my home i observed that username and pass was stored on router and login to internet automatically.Now plz someone to tell me how i can decrypt username and password of his connection?I tried on run-->cmd-->telnet 192.168.1.1
but i just made it to see his username.The password is ******.How i can decrypt it is there any way?