What's wrong with favoritism? As long as I don't neglect the other students, what is wrong with spending extra time with a given student?
I've notice some people come to Aikido and give the bare minimum (nothing wrong with that). They come to classes once a week (due to prior commitments, responsibilities or simple laziness), and they train & progress at a good steady pace. Then there are people who come to the dojo four to seven days a week. They come to class early and leave late so that they can practice and stretch by themselves and they are extremely passionate about the art. Why shouldn't I invest extra time in the person who sacrifices his time and energy for the art? Why shouldn't he be groomed for better things?
When I was in college I worked hard to become the favorite of all the teachers in my major. I got excellent grades, asked questions and had an honest interest in the subject matter. They responded by forming more casual relationships with me and pointing me in certain directions and giving me advice in my career track. Did they grade my test any easier? No! On the contrary I usually had to work harder as their level of expectation increased.
When I started in my current dojo, I busted my balls inside and outside of the dojo to make it up the ranks. I arrived early to practice my Aiki Taiso and Ki Exercises and I left late to help out around the dojo. I worked until my Ukemi exceeded most of the Yudansha in the dojo, so that I was being picked to demonstrate techniques with the Sensei. Did I start to receive some favoritism? Yes and why shouldn't I. I worked hard for it, and I was held to a higher standard then the other Mudansha.
Again, as long as the Sensei does not neglect the other students in the dojo, there is nothing wrong with him taking extra time (before, during and after class) with a student.