Then an exit node is what you want. You can set up with vpn like wireguard using port forward on your side. The raspberry would connect using static ip or dynamic dns.

You can get tailscale basic tier for free, and that will provide an easy to use solution

To me it seems like he wants to be able to «bring along» his homenet services without exposing them on the internet.

Tip for OP is to explain wanted outcome, not process to get there. Its hard to do, but gives better results

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

That’s the main difference between lemmy and early reddit. Reddit had good info from knowledgeable people, and moderation. Here it seems most are 8 years old with 0 knowledge talking shite. Voting to “prove their point”. Like downvoting your reply.