Friend can’t connect to my Diablo 2 LAN game

Since the two computers are on the same local network, as you confirmed, it might be an issue of either network configuration in general or firewall configuration on at least one of the machines.

There is a support article on Battle Net about neccessary tcp and udp ports for Diablo2. I suggest to look it through, but the core info is:

Diablo II/Lord of Destruction: TCP Port 6112 and Port 4000.

While, for security reasons, I wouldn't recommend disabling your Windows Firewall completely, it might help in diagnosing the issue. This article will help you disable it and in either event enable it again. It's also possible either an intermediate router or an additional piece of virus scanning software may be imposing yet another firewall.