You actually could try Ubuntu, or rather install Xubuntu (although the interface will be different). Personally, I'd just install a bare bones Debian system, then apt-get your way to a simple gui.
Here's a "minimal" Ubuntu install. You could do the same with Debian (the commands are basically the same). Basically do the following:
1) Download the Sarge Netinstall CD (if you have broadband, this will fetch all packages from the web) or the first two full CD's.
2) Install the system, when you get to the "Task Selector" screen, don't select Desktop or Server or anything.
3) Finish install and boot to command line.
4) As root,
aptitude install x-window-system-core icewm xdm
5) Then you can just try installing different lightweight programs. Maybe try firefox and syphleed-claws for email, abiword or Nedit for word processing.
If you want a similar interface, I would try IceWM, although that isn't a "desktop" (i.e. generally no icons on the desktop without installing something like Rox-filer).
Also, if she still uses dialup internet, you would be wise to invest in an external serial (hardware) modem, not the internal card probably in her box (which is probably a soft modem with drivers written specific to windows).
EDIT: I haven't tried it, but I stumbled across
Morphix-Light that might meet your needs...
