in business dealings, the simplest things can be misconstrued, eg. a client says, "can you teach me twinkle twinkle little stars" in one month?  and you look at him and he seems to have normal intelligence, so you say "ya". 

now, what you should have done is, write on a piece of paper, and state that under normal circumstances, if the student practises like once a day for twenty minutes, it is possible for him to achieve playing twinkle twinkle little stars with one hand, and possibly with the left hand too, on a one note basis with the chord structure/bass note for the left hand.
student should also try to have an instrument to practise on, using the table with matchsticks is also considered an option. the clue is, student needs to practise. 

yes, you get the idea.  
and yes, we've got into a little knot with a group who does not practise and insists on the teacher teaching 3 months with no pay.