providing your friend keeps any AV software installed and up to date and scans regularly with it, i doubt they will have any problems. free avg in my opinion, is as good as any and a lot more user friendly than some AV software. updates are regular too, so its well supported.
switching from NIS though will mean that your friend will maybe also wish to replace the norton firewall (as NIS includes AV + firewall) free zonealarm is extremely easy to use and configure click here also very good.
adaware is a fantastic little free spyware removal tool and i also would recommend it. back this up with spybot click here keep these up to date just like the AV, scan regularly and delete anything they find. spywareblaster click here will then also stop any spy/adware nasties from getting installed in the first place.
i use all the above and never have any problems with them whatsoever.