I looked, and it seems infiltration in Awesome Guns is the default, infinitely continuing infiltration. So once they start infiltrating, the whole region goes, over time.
This isn't something tied to difficulty levels. Those have rather
specific effects, which mostly do not have much to do with alien mission generation.
Since Awesome guns doesn't seem to be touching those, all you get for mission-related difficulty in particular (i.e. the difference between Beginner and Superhuman missions) is earlier guaranteed monthly retaliation missions. Infiltration proceeds as normal, i.e. you're taking a considerable hit
if it starts happening. Why? Because infiltration
cannot be stopped or interfered with other than some delay from shooting down UFOs. Well, it
can be stopped and even reversed with the right mod settings, but Awesome guns doesn't have those.
So, once the aliens roll Infiltration on their mission table, completely independent of difficulty level, you'll lose all countries in a region at the roughly one per month speed and the only thing you can do is delay it somewhat. I
think the once per month is actually more or less the delayed version already, but not 100% sure about that.
BTW, if your brother didn't know this, you can ding his X-Com nerd credentials. This, and similar stuff regarding retaliation, have been known for ages.