I would suggest to enable logging on her router (if possible) and to check the MAC address of all devices that connect to her home Wi-Fi.

On smartphones, tablets, and computers, it should be easy to find in the settings (but check if they've set up a rotating private Wi-Fi address; if so, disable it).

It might be more difficult with other devices: for televisions or other appliances, you might need to unplug them and check the router logs to see if a specific MAC address disconnects at that precise moment.

Ultimately, if you have a MAC address that stays connected all the time or even just occasionally and doesn't match any known device, you have a good candidate for the listening device.