Both people on your LAN can play WoW without doing any port forwarding whatsoever. The only thing that port forwarding buys you...
a) slightly faster patch updates since you will be allowing inbound Torrent connections
b) the ability to use the WoW in-game voice chat (which is UDP-based)
Personally I would recommend not doing any port forwarding at all. The game runs perfectly fine without it, patch updates are always slow anyway, and I don't know anyone who uses the in-game voice chat (everyone uses either Ventrilo and TeamSpeak for voice).