i think you did him right. Often times, a 2 hour session is discounted so that the client isnt paying completely 2 hours. And its 100% fair to make it up by adding time to the next session as a make up for it especially since because he is the one that has to leave the session and not on your end. Now, if you had to leave on emergency before a session is finish, then i agree with him that he can set the terms on how the session is made-up to an extend of course. But since his the one that left the session, it is fair that you just "refund" the time as a credit to the next booked session. So, he can book an hour next time and get 2 hours instead.