Extend Wi-Fi to second floor
This should work just fine, and gives you the advantage, if setup properly, of mostly separating the two networks. The key is that (typically) a router will have one &quot;in&quot; port and four &quot;out&quot; ports. Your router should have one of the &quot;out&quot; ports from your friend's router go into the &quot;in&quot; port on your router.
The one catch is that, unless the your friend's router is configured to separate the port going to your router from all the other ports, you may be able to see your friend's devices, though your friend will not be able to see your devices. That can lead to three types of possible problems:

You could print on your friend's networked printers. Not a big deal, as the first time he will complain and the second time start charging for paper/toner.
If any of your computers are infected with viruses, your friend's computers would be at risk.
If your friend's computers have minimal security, you may be able to get to &quot;shared&quot; files.

In addition, no matter how you set up the routers (e.g., even if you have one master router and two slaves, so that you can't possibly see each other's computers), if one of you does some nasty stuff that gets you on an IP black list, both of you will be affected.