On a mac, you can set up Network Locations. I have one set up with the pihole dns server, and have the Automatic one set to normal defaults. There's a simple script[0] that will change your Network Location based on the wifi network you connect to, so I don't have to worry about switching it manually when I leave home and I don't have access to the pi as a DNS server.
Granted, none of this answers your question directly, but a manual Network Location switch from System Preferences is a somewhat simple change that's a little less friction than a whitelist. The auto changer should switch it back next time your wife's computer reconnects to the network in case she forgets.