Macs and PHP

I'm got another person working for me on a site. He's a Mac user and for some reason, he can't seem to register as a new person. I know this problem isn't due to the large amount of traffic b/c it's a new site with 3 users max at any time. I haven't found any similar problems in any knowledge base and i'm thinking it has something to do with my cable modem connection and the fact that: 1. I'm in Seattle and he's in Cleveland (distance issue) - less likely. 2. ON the more likely side, all the crap that is running on my computer (ie: firewalls, IDS, telnet, applications). However, another developer in the same city as me has no problems registering on his Win machine. Anyone have any ideas? Thanks, Mike