Friend can't connect to my server, times out

It probably wont work on a tethered connection... Your "router", in that case, would be the phone carrier and at that point you wouldn't be able to port forward. (router/network admin password needed)
I'm not 100% sure, but I'd assume multiple devices share "clusters" of IPs on mobile networks, so his phone might share an IP with device B. But as I said, you wouldn't have access to port forwarding a carrier owned network.