A service like tailscale will solve the connection to your home net automagically. You are however stuck without routing from friend-net so you cant access homenet devices directly

You can solve this by setting up a reverse proxy like caddy on your raspi, and access home-net web-apps and services through that. Like [assigned-friendnet-ipaddress]:8444 or similar. The reverse proxy would forward this to homenet devices through the tailscale vpn