The app requires location services for pairing with the service. I’m betting it uses them for a lot more then that (though you could set permissions to “only when using the app” and prevent the app from starting automatically).
If your therapist has a Meta app, it knows you both arrived at the same place around the same time and stayed for some duration. It’ll use that for friend suggestions.
The good news is, it probably doesn’t know much more about your relationship than that, and it probably suggested a bunch of other people that you ignored because you didn’t recognize their names/faces, but yeah it’s very creepy and intrusive as fuck.