After sale of Netatmo thermostat - new owner cannot connect that thermostat

Tryin to help someone out that bought my “old” Netatmo thermostat. He managed to setup the relay but cannot connect the thermostat.
Is there anything else I should have to prepare it for the sell?
In my Netatmo account the relay is no longer registered. I don’t see how I can de register the actual thermostat.

Any help would be welcome.