ATT does not allow tethering at this point, although there have been rumors about selling an additional service for tethering. Currently, iPhone users are taxing ATT's 3G bandwidth so much , that there is no way they are going to allow it until they figure out how to deal with the traffic they already have.
Having said that, tethering is being used by the jailbreak community.
He will have to jailbreak his phone. This allows one to customize and install applications that are not digitally signed by apple. After that, you can easily hack your phone to allow tethering or buy an app which is already set up for you.
If he is interested, have him research Blackra1n and visit
iPhone Modifications - iPhone Forums