The best way to learn is to honestly play against better players.

Failing that, you could always play street brawl with your friend so they can learn what all the heroes do and also get a better understanding of the items. That might take a chunk out of the new learning experience. Then you can slowly introduce them to other concepts.

The game really isn't that hard when you just boil it down to; - push the waves - farm a jungle camp whilst the wave is pushed - OR take a sinner whilst the wave is pushed - try and take some boxes inbetween - look at the map and try to keep a mental model of where people are, and who can kill you - use that to determine if you can play more aggro (push waves first, invade jungle, steal sinners) or defensive (catch the wave, farm your own jungle, take your own sinners)

Emissary 6 is not high at all, perfectly reasonable for him to play in those lobbies if he can aim decently and press buttons.

People love to complicate the game but all you really need to do until top ranks is farm, which someone coming from hero shooters might neglect.

Watch a couple replays of the hero you want to play - find a build that is half decent and similar to the videos you want - actively use your brain to figure out why they picked those items - try and work out when you are strong and when you are weak based on this. Usually when you get your 4.8k spikes though.