So, you want the traffic to go other way around. Traffic from the HomeNet should go to the internet via FriendNet, right?
That is what I would like to achieve, yes. Since I want to avoid setting up port forwarding on FriendNet, I would need to configure port forwarding on HomeNet. The Raspberry Pi would have to act a client, and either the home server or the home computer would act as the server, and the Raspberry Pi would proxy traffic from the server to the internet.
I can’t think of how it would work in practice, though. I’m assuming in this case the home server would act as a proxy between the home computer and the Raspberry Pi, and the Raspberry Pi would act as a proxy between the home server and the internet. Unless there’s an easier way that would remove the home server entirely, that might be the best way to do it.
Besides being able to learn about all of this, this would be the ideal outcome:
I wouldn’t want to make the friend set up port forwarding or configure the Raspberry Pi himself, I want to do as much as I can on my own. And, if that friend ever relocates his residence or changes his ISP, setting it up again should be as easy as him plugging in the Raspberry Pi to the new network and I reconfigure it from my end.