A community to help people learn, install, set up or troubleshoot their home network equipment and solutions.
How are you sure it's his device? Maybe you can do a MAC vendor lookup on the unknown device's MAC address? Then you could at least find out if it's the same manufacturer as his device.
Maybe that's not my business, but why is he even in wi-fi range to begin with?
Is your ISP provided router or your own?, if it's from your ISP he may be calling, verifying the account and asking for the pass
In theory (but probably not?) he could have installed a VPN ("Virtual Private Network"), either on the router itself, or on a device connected to the router.
A VPN allows someone to connect to a private network over the internet. So for instance, I can connect to my home network from my phone, from anywhere.
However, reseting the router to factory default would have removed any VPN from the router. And changing the password would have disconnected any hidden device running VPN software, unless it was on a device you use, I guess?
Because of that, I'm inclined to believe that you've handled this correctly, and if I had to guess, I'd say it isn't him - though I obviously don't know your full situation.
Like Ansis mentioned, it's possible that this is something benign.
If your router gives the option, maybe try looking for the MAC address of the unknown device, and block that MAC in your router. If it turns out it was something of yours, you can just unblock it. But honestly, I think you're okay.
You got to show what you see.
Most likely you are just seeing another device physically connected with ethernet wire.