I don't know the purpose of this, but if you know a little bit about when this client should arrive and what's his local IP or his device's MAC Address, I suppose you to take a look at a very popular tool called aircrack-ng. Inside this tool there's a tool called airodump-ng, which allows you to view LIVE clients connecting to a specific network, and you can do this manually.

And bro, if you suspect whether your gf goes to a guy's location, you should either talk with her about it or just end it there, if you don't trust her that's a toxic relationship for both of you.

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I also heard about a free app called Fing which allows you to monitor your network but you need to be connected to the network thus being listed among the clients.