He has two DMPCs? Dear god, stop him. Explain that that is not the way to balance things. Explain DMPCs are a plague. As a DM, it is the DM's characters story; it is theirs. CR exists to help balance encounters.
Wanting to DM a game is all very well, but he should at least know the rules. Tell him to learn them or you won't play.
We're at level 20. We started at level 18, and are using mythic tiers to increase in power instead of running your usual level one-to-whatever adventure. This is because it is a continuation of a campaign we just finished that went from one-to-eighteen, with all the same characters, but a different DM. The first DMPC was his PC from the previous campaign, and is a full-caster type character. The second DMPC is new, and a bard. The bard does nothing much useful in combat other than perform, but eats up a lot of role-play.
The DM has never had much success in the way of balancing previous campaigns he ran, so throwing a high party count and a high level campaign together only makes it worse. Either combat is stupidly tedious or ends in one round.
I've seen DMPCs work fine, but that's because of the DM being able to have a character without actually using them in an overpowering way, such as having them be a support-type class, like a cleric, rather than a wizard with literally every spell. The current DM shoots himself in the foot more often than not because he literally metagames against himself.
Also, he knows damn well that if he goes overboard and horrifically murders half the party like he did in the last campaign we ran with him, we probably won't run another with him again.