GUI PGP / GnuPG application

You can also use gpgtools (https://gpgtools.org) it is got all the possible GPG tools you are probably going to need (they also have a gpg plugin for Apple Mail) and adds Services for signing/encrypting/decrypting/etc. to the contextual menu (right-click->Services).

Create a new service with automator so your wife can decrypt the file using the contextual menu in the Finder.

1. Create an Automator service
2. Service receives selected files or folders in Finder
3. Add a Run Shell Script action.
4. Add your code: gpg -d $1
5. Save the service.
6. It is now available in the Finder via Right click > Service > youServiceName.