Do I need to say anything more after you guys have posted the answers? But you guys didn't mention 2 things:
1. He can buy domain name and hosting from me

2. There are also free hosting options available like
http://www.zeeblo.com. (There are no free .com providers

)
Of course, there is a hugh question mark over zeeblo's reliability now after so many people lost so much data. But it is up again.
Secondly, there are some free forum provider where any one can have his own forum. Search Digit forum and you will find some providers.
OFF TOPIC:
Starting a forum is easy. But from where do you plan to get members? There are thousands of forum out there already!!