Hello,
It might be possible. I’m not sure how to go about explaining.
You can add more managed routes to your zerotier network and they will get applied to everyone’s machine. The routes would look like [ip-address-of-a/32] via [ip-address-of-c], [ip-address-of-b/32] via [ip-address-of-c]
You’d also have to enable routing on your (C) operating system. I don’t remember how to do this in Windows.
There’s a high chance this won’t help much.
It’d be better to focus on getting your friend’s routers if at all possible.