With newer versions of Steam, it’s just handled automatically as long as you enable it, you can download from other players on the network. True not all games are on steam, and this probably works well for massive LAN setups, but I tried and failed to set this up, it’s a bit of a pain and personally found it not worth the effort.

You can trust the person, without trusting their technical skills, such that they haven’t inadvertently installed malware on their own devices.