I know nothing about Macs. On Windows, if you copy the catalog folder to an external drive, you can simply double-click on the catalog.pse25db file found in the copied folder, and it will launch that catalog in the Elements Organizer. Otherwise, you would go to the File>Manage Catalogs dialog and select the catalog from a custom location on the external drive. I assume something similar can be done on a Mac.
You would not need to remove the catalog from the Mac internal drive, but be aware that you will now be using two different catalogs. It will be somewhat difficult to keep them in sync. Work performed on one catalog will not carry over to the other. So, if you decide to use the external drive catalog it is best to always use that one. If your wife's computer is Windows, I'm not sure whether the same catalog can be used by both Operating Systems. I am going to ping @Jeff Arola , another community expert, who will know the answer to that question.
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