@stefano
I'm not surprised. Some years ago, when I installed Blokada, I discovered that my old #Motorola (#Lenovo) phone contacted graph dot facebook dot com every time I connected my Bluetooth headphones. (I assume the reason it did this was to discover which other devices belonged to me, all the better to spy on me.) No #Facebook apps were installed on the phone. Eventually, by learning how to use the Android Debug Bridge (ADB), I found a Facebook process that I couldn't disable or remove.
Unlike your customer's phone, my phone displayed nothing in the Android UI: there was no clue that anything creepy was happening, and no way to disable it.
I solved the problem, kinda, by instructing Blokada to enable a blocklist that blocked all Facebook domains. You could suggest that your customer do the same. But, still, I hate the idea that Lenovo was using my battery power and bandwidth to spy on me with no notification, no consent, and no opt-out.
I'm pleased to say that my #Fairphone doesn't have this creepy spyware and doesn't contact #Meta behind my back.