Although there is the related question, as to whether rank can continue to go up if there is some true physical reason why certain ukemi are not safe for one. I know someone whom I think is being held at 4th kyu by the sensei until ukemi attitude improves. This person is not necessarily afraid of ukemi (although one should always be a bit afraid of injury), but even after several years seems unwilling to take it seriously as a part of training. Makes it difficult to practice with this person.
I don't want to hurt, embarass, or refuse to train with this person (actually, I do WANT to I just don't). I consider this person a friend; still, subtle remarks from myself and others have not had any effect. This person will not be able to progress very well. I'm not familiar with the tobi-ukemi by name. I do know that it certainly helps to take koshiukemi if one wants to learn various koshinage. Does one have to "land a certain way"? I don't know; but one should allow oneselves to be thrown at all. Me, I know how I'd like to land before the throw, find out how I have to land during the throw (connect those two on the way down) and critique myself once I'm on the floor. A bit of fear is always present, yet I trust myself and NAGE.
This person must find it within themself to decide to learn. The desire to help kohai is good, but I think the only thing that can work is a candid heart-to-heart. If the person listens, problem solved. If not, well, I don't have it figured out.
From a selfish standpoint, what can I do when this person wastes my training time by "bugging out" because of "fear of injury"? Not just breaking connection and refusing to roll, but actually leaving the mat. I'd prefer to work with someone who is there to learn. It would be cold (even for me) to say, "I'm here to practice with those whom want to learn." I try to always see the best in people (it is often difficult) and believe each time that "this time it will be different." All I need is an excuse...