Adsl routers can sometimes be set up as switches, but as FT pointed out, this setup provides little or no additional protection. My view would be to ensure he has a good av package which is able to update itself automatically and also to ensure that the software firewall is on, and windows is also set for auto updates. As Gec says, some users are capable of getting themselves into difficulties regardless of the safeguards in place, but this is sufficient for most people.
Jabo
PS a cable router will provide more protection, they're pretty cheap, so it's the next logical step