Hmm to make sure its just you putting that launch command in right not both of you. If not that it might have something to do with your firewall but unfortunately can't help you there. Arma 2 uses 2302 as the default port as as long as you change it to anything but 2305, 2307 you can try that. You could technically change the port to 2303 because it goes up in values of 2. I don't think the port matters, but I could be wrong.