Antivirus Software

[Chuck Vose]

If they're running a mail server (which they shouldn't be using Ubuntu
for incidentally) amavis is a great antivirus scanner. The only
viruses that exist for linux prey on poorly configured systems, so if
your friend is doing a stock Ubuntu you should be just fine without
antivirus.

Everything in linux is free, if it isn't, don't use it. There's 37
alternatives for any program in linux; no reason for you to ever pay
for a linux program (unless you're paying for support).

-Chuck