Creating a separate network inaccessible from the main LAN

My friend has rented an office inside another, larger, office. They let him use their internet via ethernet off their main switch. He has no physical access to that switch or any ability to configure it or the ADSL router. He just has a single ethernet coming off it.

He can see all of their network and visa versa. He obviously doesn't want this and wants a separate network for his own PCs, including wifi.

I've done this before by adding a router but it's been ages and I can't remember exactly what the upshot of it was - if I stick that cable into the new router's WAN port and create a new network, will his network be inaccessible or there's?

What's the best way of doing this? Thanks