I might be wrong but maybe a static ip for each pc, for example set your pc and your mates pc network ip's to the following.
You 192.168.0.2 Friend 192.168.0.3
Do not set it to 192.168.0.1 that will most likely be the ip you use to access your router's control panel. the methods for setting an static ip are different for each os but failry similar, should'nt be too hard to find a guide.
Hope it works if not atleast you've got your pc's setup to handle multiplayer games a bit better and port forwarding will be easier aswell.
Haha, set static IP for each PC Maybe you will also suggest them to use IPX? We're in 2011, not 1998... Well, cant blame you for that, since its Crysis 2 we are talking about.