Why can't I connect to local devices via VPN when IP subnet is the same between both networks?
In your Mac's network settings, the local network is probably set to 10.0.0.0/255.255.255.0, this means that all IP addresses from 10.0.0.1 to 10.0.0.255 are local. The option &quot;Send all traffic over VPN connection&quot; does not help here because it affects only non-local traffic.
What you might try (but I would not recommend it) is to change your Mac's network setup to manual, then change the network mask, so only your friend's router is left in the local network. Assuming the router has 10.0.0.1, then the mask should change to 255.255.255.254 to route all other IP addresses through the VPN. Note that this makes it impossible to reach any other device in the local network. Note also that this setting cannot work when you are back in your own network. Finally, note also that this is a theoretical solution which I haven't tried, and it may well be that it causes other problems that I'm not thinking of.
The correct and recommended solution would be that you change the IP range of your local network to something more unusual that is unlikely to collide with other local networks, like for example 10.99.111.0/255.255.255.0. This is how I solved this problem, and I've never had any network collisions any more, even though I've used VPN from many different networks in other companies, hotels etc.