Android - how to find out the gmail account for a flashed device?

Question

Android

HTC phone available. The owner is far from technology.
When auto-updating - the person turned off the phone and that was it for the phone. He didn't turn on anymore.
The service center, apparently, made a Hard Reset and uploaded a fresh firmware.
The person is very sad behind the contact book and claims that he had a gmail account on his phone and contacts were synchronized. At the same time, he does not remember the account name on gmail and the password either, but he claims that he can restore access to control questions.
Therefore, the question is - how to find out the address of the account that was on the phone before the hard reset, or where to turn about this? The phone is white and all documents are available for it.
Thank you.

Answer 1

If the account is linked to a mobile number, or if you specified a spare mail during registration, you can try to restore it here

Answer 2

If this mail was not used anywhere else, then it seems like nothing.