aikidoc

I think the most courteous thing to do if he is going to leave is to let the former sensei know. Especially, if he is testing-this will get back to his former sensei.

As to justifiable, I think people have to make their choices. Some sensei for whatever reason don't seem to recognize students appropriately. Sometimes other students are put ahead for whatever reasons. If the person feels they are not being treated fairly, although it is subjective and should be discussed with their sensei, then they are not happy and need to seek a situation that they feel recognizes their skills and efforts. It's a tough call.