Well bet him a night on the town he cannot find any OS X viruses.
Depending on how you define "virus", you might lose that bet.
There are no viruses (defined as self-propagating malware) in the wild for the Macintosh.
However, if you use the term "virus" to refer to all malware (as most Windows users do), then your co-worker is absolutely correct. While the amount of malware for the Mac isn't in the same ballpark as for Windows (roughly about 30 mostly innocuous or extinct examples for the Mac, compared to well over a million for Windows), it definitely exists. Here is a meticulously and constantly updated list:
The Safe Mac : Mac Malware Guide
Here is a thoughtful article about folks who go around saying that there "are no viruses for the Macintosh":
The Safe Mac » “There are no Mac viruses”
As for the "best" anti-virus software for the Macintosh, there are multiple ways to rate such software. If you rate it by how effective it is at detecting known malware for the Macintosh, you can do this objectively because Thomas Reed yearly does a huge comparison test of all of the programs available. Here are the results of the latest test:
The Safe Mac » Mac anti-virus testing 2014
You can download the results and study them more closely, as a PDF file:
http://www.thesafemac.com/downloads/malware_scan_results_2014.pdf
However, another factor to consider is that all of the fully interactive anti-virus software programs have, at one time or another, been implicated with causing nasty software conflicts, slowdowns, rotating beachballs that are persistent, etc. Since OS X includes its own (non-publicized) anti-virus software:
XProtect/File Quarantine
File Quarantine - Wikipedia, the free encyclopedia
OS X: About the "Are you sure you want to open it?" alert (File Quarantine / Known Malware Detection)
Third party commercial anti-virus software may be more trouble than its worth.
Most Macintosh users do completely without any sort of anti-virus software, yet you almost never hear of an actual first-person believable account of a Mac user becoming infected with malware. (I include the term "believable" because many users, especially new switchers from Windows, automatically blame any problem on "a virus", even though that is almost never the actual cause of their problem.)
The choice to use or not use anti-virus software is up to you. You may want to read through:
The Safe Mac » Mac Malware Guide
Which is a Web site written and hosted by someone not in the anti-virus software industry, so the information there tends to be reliable.
I can tell you that I've used one of the top-rated anti-virus (AV) programs, Intego's Virus Barrier, continuously ever since OS X was released, roughly 13 years ago. I use AV software, not because I think that I need it (I don't think that I need it), but because my work requires it. In all that time, my AV software has never flagged anything that I actually needed to be protected from.