Hey if her desktop still works and it's super cheap- or free, it's worth a try.
Just don't use it for any secure banking or put any important info on it.
Just in case it gets a bug or malware.

There is a internet/network set up section on XP in the control panel that takes you through how to connect another computer to the one that is connected to the modem.
Look for Network Setup Wizard in control panel.
That would be the simple & cheapest way to try it.