Log in as yourself (admin), get info on the folders you want to give her access to, open up the "Ownership & Permissions" tab and grant her access that way. It would probably be the easies to give "others" read and write access, as well as any other groups that her ID is part of. You can then click the "Apply to enclosed items..." button and it should propigate to everything within the folder.
It would be easier if you just made your wife an admin of the computer as well. Or do you not trust her to not f*ck up the computer (either on purpose or by accident)??