Hi,
I have several wrt54's all talking to each other so yes it is possible.
What you need to check is the your wrt54 is in clientmode and talkinf the the other AP. Then plug your main cimputer into one of the ports. Ensure that the wrt has dhcp enabled or yo are using a static adress on your main machine and wrt that is in the same subnet, ie all on 192.168.1.x.
Your gateway should point to your mates AP and other than that it should just work!!
Can you ping his AP from your main computer? Can you ping the outside world?
Wookie