Strategies To Avoid Virus Spread In A LAN?

My friend is coming over to get his pc fixed because it just happens to be so that his internet was shut off because he forgot to pay and he isn't responsible for the bill in his household. He always has a trojan or two in his pc and because of that I'd like to somehow "quarantine" him in the LAN so that his trojans viruses etc. can't infect other pc's in the network. I was thinking of attaching another router to the first router, so that it makes a new internet connection, and he wont connect to my main wireless network, so that he cant infect my machine through lan? TLDR: I want to make it so that it is as if he was on a far away modem in another house, so that he cannot infect my machine through lan, but still having him access the internet through my modem, by possibly adding a second router that is attached to the first router.