I would discourage the use of IRC, particularly if it is going to be used over the Internet. The Honey Project has a lot of information about the perils of IRC. I just went to their web stie to see if I could find anything that is a quick read and yet descriptive of some of the problems associated with IRC. Here is a link:
http://project.honeynet.org/scans/scan27/sol/grenier/
There is a lot of information about IRC at that web site. The problems listed on the page that I have referenced above range from nuisance level disconnects to virus spreading. If your client is going to use this to communicate with customers then the nuisance level problems would create a bad image for your client while the virus spreading problems would both create a bad image for your client and would be a genuine threat to your client's customers' computers.
I looked around for a program that I heard of a while back called Jabber. This is not a free server program but a free open source work alike is available at:
http://open-im.net/en/
This protocol is supported by numerous open source chat client software such as Gaim. More information is available here:
http://sourceforge.net/search/?words...of_search=soft
In summary I would stay away from IRC software. Naturally I hope that this has been useful. I spent about an hour doing the research.
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Addendum: Someone started a thread about open source chat client software after you entered this question. You might find that useful.
http://www.linuxquestions.org/questi...d.php?t=406551
IRC anyone?
