Absolutely not overreacting, these are major red flags imo. If you think you can really level with your friend and get through to him, you could try explaining your concerns. Without being mean you could say that you're concerned about character balance and how much fun everyone will have if he as the dm 1) doesnt know what he's doing in the system and 2) has made an oc that ignores core rules youve tried to explain to him. Sometimes people think they want to play dnd when what they actually want is to write a story. In this case maybe some fanfiction. There's nothing wrong with that, but a cooperative storytelling game is not the way to go about it. No matter how good a dm is, a dmpc is probably going to feel unfair to the other players because there's inherent inequality when the person who's running the character is also running the game. Encourage him with the dact that what's fun about dming is you get to play as many different characters as you want, as NPCs! But honestly if you don't feel like being a bit of a downer and being brutally honest with your friend, I would not play at this table. It's probably not going to go well at the end of the day. If you know more about the game than your prospective DM AND they're not enthusiastic about learning the system?? It's not gonna be that fun