Buddy and I new to Civ, trying to save his poor soul

Friend and I are playing our first Civ VI game together, and we both have one win under our belts solo so far. We’re playing as allies on opposite sides of the world on a small continent map (huge size). I got absolutely wrecked by barbarians (we did clans), he got his expansion pinched by city states (we set them to max without thinking), and we were just starting to make a comeback when his capital got flooded and we realized he hadn’t upgraded a single district. We just started the Modern Era and he’s falling behind because his production, science, and culture are all way low. What suggestions do you have? Right now we’ve managed to trade a pair of cities with each other so he can spam short trade routes into my cities, but since he doesn’t have upgraded harbors or commercial districts, he still only has a couple traders available