Not sure how to test it, but he's never been kicked from any game at all (not just arma), he doesn't lag on any other games so I don't think so, but I guess that it is a possibility.
Do you know whether I could just send him the mission file over skype or anything?
EDIT: Just found something...
EDIT: His pings basically spiking for not even 0.1ms, his phone connection (tethered works fine), but when he switches back to his main internet he can play for a little while but it will inevitably kick him when his ping spikes, absolutely no other game lags, and it's definitely just a very...