It depends on where he is at in the learning... It's been a long time, but this is what I did.
For any new player....
1. Start in classic, once you have played each role/ understand how to play them, move to ranked practice. Rinse and repeat-play that mode until you understand each role and then you can move to other game modes.
As your role comes up, read about it in the wiki as the game loads and during the nights. Stay only on your role page, if you read all of it and still have time, read it again and truly learn it. The best way to learn a role is by playing it.
2. Have the role list and know how it fits together. You can definitely use helpers for this. Some people choose to use a notepad/word doc or something like that. Some would rather it be physical pen and paper (I preferred that because i could keep up with the chat better.) But I actually took it a step further- Lifehack incoming~
I wrote all the roles on a sheet of paper and put that in a picture frame. Using a dry erase marker, I kept track of the game. Here's an example. (Its old and the role list has changed xD )
Circled known roles, lines meant the role was dead. Squiggly line under a role was a claim and I wrote player names when I knew them. In my pic, it shows we had a dead sheriff, we knew we had a transporter (people claiming they were trans, or maybe I was trans'd), a dead doc, a vet (who killed on alert), and a vig (probably attacked someone)....and so on... We also had a player named Chance claim to be survivor. This helps with role claims and piecing the game together as a whole. I could quickly learn some overall social game skills this way.
It is also helpful to make a list of the invest results such as:
vig/vet/maf...
GF/arso/BG...
You can add other interactions to this like who is immune, what roles the sheriff finds as suspicious.
3. Play your role and keep a will. If you are town-write your role then each night write the night number and who you clicked. If the system prompts you anything, write that too (TI results, doc healed). Don't forget to write stuff that happened TO you. If you are evil, tell your team you are new and still learning, ask for their help. Pick a role to pretend to be and then commit to it. During the day, you need to act like that role. Be sure to keep a will and update it with your fake nights info as needed.
Doing those 3 things will be a good start to the game and at least allow anyone to play it without screwing up the game for others. Remember this is not a game that can be learned overnight or even in just a couple games.
For those teaching new players (which is also something I did alot)
Let them play. You don't need to be constantly telling them what to do or how to do it. The best way for them to learn is by experiencing it. I don't know what you are trying to teach during the game, but you can probably cut back on it. Your friend needs a chance to develop their own methods and not just be a copycat of you. There isn't on way to play this game and maybe your style doesn't fit your friend's style. Take a few minutes AFTER each game to discuss what went well, what went bad, why they got hung (Despite being town) or whatever game highlights you want. Then add some "for next time" strategies.