Well, I never could get IEs4Linux to work on Intel Mac. Never could find the right files. My mind was all jumbled up with x11, Darwine, Wine, apt-get, Fink, ./configure, Apple Developer Tools, and dead links.
Stupid solution staring me right in the face the whole time (or should have been staring me in the face!)—Opera! Opera did the trick. The newest version of Opera has not only a user agent switcher but also a user agent masker, which, as far as I can tell, tries to render the site as IE would (not just pretend to the site as if the browser is IE).
I don’t know for certain it’ll work fully with my mother-in-law’s IE-only website, but it looks like it might. We’ll see what happens when she logs in.
Opera, you’ve come a long way, baby.
Edit: Nope. No go. It appears to work but then hangs indefinitely on login. I did a bit more research and found that the site she uses calls on ActiveX. Weird that it works on IEs4Linux on Ubuntu, then.
Back to the drawing board…