Certainly strange, if it were as reported I would have expected to have see in impact all users on the network.
But out of the house it is possible with a weak WiFi network password it could be possible for someone else to connect to the WiFi network.
I would check his Network Inspector settings in the 2 images I posted and ensure that the protection levels are monitoring connections. That said I don't think that when he comes back in his system would have to effectively reconnect to the home network.
It might see a visited and unknown network, as in my first image, but given the range of WiFi it shouldn't retain that connection and be able to use it. As I have said the network inspector is essentially a local service to prevent unauthorised access to your WiFi.
I'm at a loss as to why he (?) might think it is the network inspector as it is a local service.
Avast attempting to gain access to his device's data and that attempt causes significant internet traffic at his end.