Now, i have a similar situation where we only have 1 router. This is what i did
Note: You'll need to let your friend to connect to your router
Get your friend to allow you to connect to his router.
1) Got to START/CONTROL PANEL/NETWORK AND SHARING CENTER.
2) On the left, you'll see ''Manage Network Connections'' Open it.
3) Right click on either your Wireless or Wired connection (Deoending on how your pc connects to the router) and click properties.
4)Click on the sharing Tab and tick the ''Allow other comuters to connect through this cmputer'' box.
5) A drop box might appear. If not, its ok, if it does, set the drop box to your ''Local Area Connection''
6)Plug the xbox into your oc with an ETHERNET cable.
7)Repeat steps 1-6 on the other computer.
8)Run the same game, and with the same updates, then choose a System link game. Set the game up as usual, one as the host, the other as the slave.
Hope this helps you!! Good luck !!!
P.S. Not sure if the XBL headset works on a system link match.