Where does the Apple app Contacts store its files in Mavericks

The path is ~/Library/Application Support/AddressBook/ on both Snow Leopard and Lion.

Note that you can't see the Library in your home folder (as it is hidden by default). You can type in chflags nohidden ~/Library in Terminal to make it appear permanently, or hold down ⌥ Option while in the "Go" menu of Finder.

Using Numbers would work as you say it syncs when ever a change is made. The benefit of this is that it is customised to your wifes needs, i.e the filed Name, Instrument, etc, ect are what she needs.

Using Office to go adds a few unnecessary steps to the procedure, with iCloud everything just syncs.

If she were to buy a database package it is not likely to contain all the correct fields she requires and would contain a lot more that would be useless, so a bit overkill really.

However the benefit of having a database app like Bentois that all your contacts details are clickable on the app, so if she is looking for a bass player she will be able to search and then click the contact number to call them. However this has a much larger cost than just buying Numbers for iOS.