How to disable "show password" on my wireless network

I share internet (through my base station) with my cousin who lives upstairs. In system preferences when you go to Network/Advanced/Airport, highlight my network and click on the little pencil to edit it, you can see "show password" which reveals the password to my cousin. I don't understand why my cousin can see my password, but my own computer has that option disabled. I see the option but it's gray and unclickable. Is there a way to disable the "show my password" option on my friend's computer?