I assume that this person already understands the stave and can, slowly and methodically, translate between a stave position, a note name, and its position on an instrument (that is, which key to play, which fret to hold, etc. depending on the instrument).
I assume this because they're simple concepts.
What he needs is practice; and to encourage him to practice, motivation.
The motivation is to realise the usefulness of being able to do these translations at speed. There are a lot of musicians who can't read sheet music, but almost all musicians have a need to communicate using note names, so your pupil should understand how useful it is if:
- He can play a note, then immediately tell his band-mate "That's a B"
- When his band-mate says "Play a B", he can play one immediately.
That's one-third of the circle in the illustration.
Since he has asked you to help, your pupil clearly wants to be able to work with the stave, so he must see the benefit already, but it's worth taking some time to hammer home what that means:
- He can see a note on the stave and say "That's a B"
- He can say "I want to write a B", and put a note on the right line of the stave.
- He can see a note on the stave and play it on his instrument.
- He can play a note on his instrument, and put a note on the right line of the stave.
That's two-thirds of the circle in the illustration. Clearly you can "cheat" by only learning two-thirds of the circle. For example, to play a note read from the stave, first translate into a note name, then find the instrument position for that note name -- but he needs to understand that the end-goal is to not need that.
The purpose of all this is to motivate him to practice, because he knows what skill he's aiming for, and how each practice exercise helps toward that goal.
So we have 6 skills. He may decide that some of them are not relevant to him (yet). For example perhaps he wants to sight-read, but doesn't want to write music; that would remove two skills from the list (and actually, for many people, having learned to read the stave, writing it is a doddle).
Now, practice, practice, practice. I think sometimes we forget that the word "exercise" when used in schools, textbooks and musical pieces, is the same word as that used for physical exercise. When you exercise in the gym, you strengthen your muscles. When you do maths exercises, you strengthen your maths skills. When you do musical exercises, you strengthen your musical skills.
Unless your student lacks motivation, I don't think you should need to actively participate in his practice. He needs to sight-read music -- slowly at first, by manually counting the lines to each note if necessary, but with practice, faster and with more complex pieces. He needs to speak the names of notes as he plays, and as he reads from the stave. If he wants to write music, he should transcribe pieces he plays by ear.
The more he does it, the more fluent he will become.