I didn't think Messenger cared about your IP address... only if you were connected to the internet or not. What version are you using? Windows Live Messenger 2011? If so, you have to click on "Options" on the main sign-in window, as there is no "Tools" option like in previous versions.
Aford, that is Windows Messenger from Windows XP... something not included with Windows 7. She has to be using some version of Windows Live Messenger.