Does he have a static IP to his router WAN side? If not he will need to get with dynamicDNS and they will do it for him. Otherwise, he will have to contact the people with the new IP of his web server as it changes. And a lot of providers look for web server on non-dynamic connections. Did I mention I run US Internet Technoligies?
Basically, he needs a static IP or dynamic dns services. That will keep his router on one IP. Then static map the web server machine on the lan side, like 192.168.0.2. Go in the router, assuming you have the password, and it will port forward. Forward port 80 (like mentioned) to that IP; the 192 one. You mentioned IIS, make sure this machine has the current microsoft updates or he is just asking for trouble.
Also Adsl is slower on uploads (outbound), so the people that hit his server will hurt his DSL speed, it is fast IO to him, not from him.