It's easier if you host through the windows pc.
There are three ways to do this:
ADHOC:
First step is to make an ADhoc network from the windows pc.. assign an ssid and a password and connect to the network as you would normally (this works with wireless).
Next step is to create a server normally ingame, and have the other player connect to your computers adhoc ip adress (do "ipconfig" in your command line to figure this out), afterwards type the following in the ingame console: connect 'adress from ipconfig'
INTERNET:
If you want to host a game this way first you must make sure that you have access to the port forwarding option on your router, if you want to know how to do this you should search online as it's different depending on your router model... The default xonotic port is 26000 so you have to make sure this is the correct port that is open
Next step is to open a gameserver and have your player connect either through the server list (not sure if this works) or search for "my ip" on google and have him type "connect 'your ip'" in his console
HAMACHI:
This is the last way I know of that you can get a game going, first you will need to download "hamachi", use google for this..
After you've downloaded/installed the application on BOTH computers, have one of you create a hamchi network, set a password and send the other player the details so he can connect.
Finally on your network screen you will have an ip address showing below your network name, send this over to the other player and have him type "connect 'hamachi ip'" in his console.
I apologize in advance if I've made any mistakes.
All of this assumes you are NOT on the same network, if you are you simply need to:
-find your local ip address (ipconfig in terminal)
-have player "connect 192.168.0.xxx"
Sucks at weapons