Pick the Elhonim. They have a ritual that can drop the AC. They are kind of the anti AC faction. Sadly the AI is less than stellar at that job but as a player you can do it.
Another option would be playing as a Barbarian faction - Either the Clan, Doviello with Charadon or, the strongest and likely most unexpected one for your friend, the Infernals themselves! Civs with the "Barbarian" trait won't be attacked by any of the barbarian-civ units, including the horsemen. The Sheaim aren't and so they will actually get attacked!
You can play as the latter by being the first one to unlock the Infernal Pact technology. Ideally you want a good economy for that, Lanun (with Hannah) and Kuriotates are most likely the easiest to rush it. Sheaim themselves lack any sort of economic bonus, so once you get the basics down, you can reach whatever you want before them when having an economically superior civ.
The Sheaim's two main strengths are: 1) Rushing with pyre zombies, but that doesn't seem to be what your friend is doing. 2) Many cities with planar gates and a high AC. This however is a slow thing and raising AC in particular takes quite some effort. 5 AC counter value per 300 Hammer ritual is very expensive, especially since map size and civ count will often have you need something like 250 AC value to actually reach 100 (displayed on hovering over).
As for economy, take a look at this channel:
https://www.youtube.com/channel/UCQPjXaLpGKmRm05QGkhQ4Ww/playlists
He does some good videos at high difficulty vs the AI, which should be mostly applicable to games against humans.
Or, just tell your friend that you are impressed by his skill and that he shall teach you the game, then learn what he does and counter it.
