Messenger Plus! Live adds a command line parameter to Messenger which allows you to launch with a particular skin, so you can create two seperate shortcuts, one with the skin and one without.
The locations the shortcuts need to point to are as follows:
(these need to be changed where appropriate if Messenger is installed in a different location)
code:
"C:\Program Files\Windows Live\Messenger\msnmsgr.exe" /PlusSkin="Sweeney Todd"
"C:\Program Files\Windows Live\Messenger\msnmsgr.exe" /PlusSkin=""
Now you just need a way to make sure your mother only launches the bottom one