icloud location on Yosemite?

A friend of mine upgraded her Mac to Yosemite, and now it wont let her sign out of her old mobile.me account. Somewhere in the mac there must be the files that control this (obviously since creating a new OSX user will allow a new apple ID), but what files would handle this login that can be safely removed (or edited) that would allow her to sign in with her current apple ID/Icoud account?