I think the idea behind this was for him to get this guy's ip address without him knowing.
If you guys plan on sending him to a site to figure it out (and him knowing that he's giving it out), you could just send him to www.whatismyip.com
Or as said you could go to the command prompt (go to start, then run, then type 'cmd'.. which will pull up a simulated dos prompt in win xp. You can then have him type ipconfig /all
but you see, the problem with this command is that if he's established in a LAN (which is the case in many home networks or cable modem/dsl) then ipconfig will just output the local ip address.. which will probably be 192.168.0.1 or 10.* , and that won't do much of anything for you.
Or if you're trying to do it without him knowing about it.. just think creatively and find a way to get him to establish a connection with you, and then check 'netstat' in dos.
As said, there are hundreds of ways. pick which one works for you.