How are you doing it?
I'm not entire sure if OS X has drivers for that printer by default (if not, just install the latest GIMP drivers -- a lot more lightweight then the **** you get from the manufacturers)
Then in XP under your account make sure the printer is shared on the network -- you need to enable all the file and printer sharing crap etc first obviously..
Then on her MacBook under the printer prefs, click add, and the printer should show up under 'Windows' if you have everything configured properly on the XP side.
If you have any questions, ask.