Get the same friend back out LOL :-)
First of all, look at the IP address the first computer has. That's done by pressing the Window symbol and "R". It opens up a box and you type CMD on the line and press OK. It opens up a new window and you type the command IPCONFIG and press enter. It will spit out a bunch of numbers, but the only 2 that matter is the first line telling you what address the PC has and the 3rd line telling you what the address of the Gateway is. Generally the Gateway is in the form of 192.168.2.1
That's the Belkin default but it may be something different. Now, the real question is what happens when you try to connect to the network from the new computer. Does it show your wireless network and is it secure? If it is, you're going to have to find the password. When you try to connect to the secure network, it will ask for a WEP or WPA key.
Go to the working computer, open Internet Explorer, and on the address bar type http://192.168.2.1
or whatever gateway address you got above. It will open up a new box with LOGIN in the upper right corner. If your "friend" didn't set a password, you'll get right in. If he did, remember the first line.
Outside of that, finding the security key should be easy enough to follow if you open this tutorial http://en-us-support.belkin.com/app/answers/detail/a_id/33/p/386
A WEP key is 10 or 26 hexidecimal characters (0-9, A-F) not case sensitive. A WPA key can be anything from 8-63 characters and it is case sensitive so MyKey is different from mykey or any other variation.
Your last resort is to reset the router, especially if it's a DSL. You will have to do a lot of extra programming and it usually requires the original paperwork for addresses, login, and password to the inernet to set up.