As far as I know, win nt doesn't have a built in firewall. Norton might have a firewall built into it, have him check that or just disable norton all together for now and then try it, remember to turn it back on though. Also his isp could be blocking the port that battle.net uses.

edit
I got bored so I went and found this for you.
http://www.blizzard.com/support/?id=awr0222p1
/edit