The incas have the possibility to while mining gain wealth.
Suposing that you ain't playing with infinite resorces I think that your friend's tactics may be to buy resources at the market to counter the lacks he may have.
If this is the case, than perssonaly I would choose a good economic army, and I would choose my tactics wisely, I don't play DM so my opinion counts for what it counts, but try maybe using the Egyptians and try to accumulate as many wonders as you can to try to economicaly overpower him, you also have to know how to use your military tactics wisely, but that is exactely what many times decides a big battle, and therefore many times the game... try maybe understanding how he attacks, study the units he most uses, and than build the better ones to counter it. (Just my opinion).
Good Luck.