I've settled on
TeamViewer. Free for non-commercial use.
There's a full app and a "temporary one," but after installing the full app she'll be better off in the long run. You can email her the direct download link to either from that page. Just remember to send the URL as a
clickable link for her so that she doesn't have to copy/paste it into a browser.
She'll need her OS X password to install it. Don't worry about the specifics, because of course after it's done and she lets you in you can move it into Applications for her, put it in the Dock or whatever. Runs on Tiger as well.
The crux of how it works is, she launches the app and it gives her a code. She tells you the code over the phone and you input that code into your copy of TeamViewer. The resulting app appearance is a little cleaner IMO than what logmein.com provides, although the latter has the option of not needing her to give you a code first I think (but more stuff for you to do to get in if I recall.)
Not a guarantee, but I also think performance is a little snappier on slower connections with TeamViewer. It's just the two of you running a dedicated secure session within the apps, no web browsers to potentially introduce other variables.