My point is that I would not give up my body 100% and give my nage everything I've got if he/she doesn't at least try to put themselves on the edge. I specifically look for people to train with that are going to work hard not only for me but for themselves.
I practice giving 100% to each person I train with for my own learning and theirs. To do otherwise does not benefit anyone or the general feel/energy of the dojo. My sensei (whom I respect immensley) often takes ukemi for students and I believe this is one of the best ways to learn. A student's own technique improves when they feel good, connected, ukemi in response, whether from sensei or sempai or kohai.
It is important to remember that everyone has a different background and this person might have very good reasons (physical and/or emotional) that make it difficult to take ukemi. Please be compassionate (I agree, great word choice) and don't avoid him on the mat. See it as a learning opportunity for yourself and a chance to benefit a fellow student as well. Of couse, make sure you also train with people who have great ukemi, because they will also help you to advance.