He obviously has a Mac, and wants to make sure his pages run the same on Explorer. To be honest the differences in what FireFox and IE6 support are minimal, it's more about the way they do them.
Neither are particularly XHTML Compliant, let alone even close to CSS compliant. IE7 seems to be the closest browser yet, but until everyone switches over to Vista; that'll probably be a minority.
This said, if it does run in IE5 then it'll look identical in IE6. The difference between the two is minimal when it comes to what they allow website wise.
Most of this will no doubt come down to visuals really.. if in doubt then just upload a site and/or allow your ISP to be viewable online (with HTTP access) that way you can ask someone with a Windows PC to check for you and report any issues.
I've found the most common issues end up being DIV, Tables and CSS Fonts. Everything else seems to work the same.
If in doubt, just move to Flash... works identically in all browsers
Intel Pentium-D 2.8GHz, 512MB DDR2 433, Ati Radeon X1600 Pro 256MB PCI-E, Windows Vista RC1 / XP Professional SP2