This. … https://support.fairphone.com/hc/en-us
You can’t just send it back without them approving this. There’s a process to go through, which starts by contacting support.
Not necessarily, as the phone is repairable for the most part, but it’s one possible result of the support process.
How did he figure?
Did he take the modules apart or at least rearrange the connector of the display module? (This doesn’t void the warranty.)
If you happen to have fairphoneangels (<- see link for info) with Fairphone 3 experience in the vicinity, they perhaps could take a look.
Else …