MacFixIt, right now, has an article by Ted Landau about Screen Sharing, which would be all you need...except you really need Leopard on both machines.
However, many posters suggest several alternatives, some of which should work for you. Be aware, of course, that her machine will be no speed demon! The biggest problem I see is that she would have to install the app on her end. Of course, assuming that her Internet connection is MIA, none of the apps will be of any use...
We can all certainly understand the limits on what can be done by telephone as well as the enormous amounts of patience required to actually do the trouble-shooting. Short of just getting her a 'new' machine, it might be much easier and quicker to have her simply pack up the machine (and everything attached) and send it to you!
EVen if you get her a 'new' machine, I'd suggest taking delivery first and setting everything up yourself, including automatic backups to an external drive! At least with a newer machine, one capable of video iChat, you can actually see her screen and suggest what should be done...