We are talking about a generic member of society. Is your intended answer that he should absolutely do that if costs of saving him outweigh his productivity?
I'd say it's up to him and his moral system.
I think you don't seem to understand that in these ambiguous cases, I think people should just decide for themselves. I put forth my version of rationality--as it pertains to me, and the people I care for. It has nothing to do with what everyone else does, so far as I am concerned.
If you are not telling me that, at least in this particular case, you wish to view the world in your own way, regardless of logic, I can accept that, actually. I would not be able to tolerate it, and I might think it unwise, but I think you would have that right. If that is who you are, and you want it that way, I have nothing to say against it.
Maybe I just don't love them enough.
Or maybe it's that I thought about that guy, and how he might have kids who needed him, and how he really wasn't up to giving whomever bone marrow anyway. And I thought about what kind of life you'd have to put you in a place where you'd let someone else die when you could have saved them.
Oh, and I thought through the scenario with a young, pretty woman, too, who had cancer (nothing she could have possibly brought down on herself)--a single mom--and wondered what sorts of responses we'd get then. Or a young man who wasn't really old enough to have thought much through, who was young and angry. Or a gay man who had contracted syphilis and left it alone for a while. Or a renowned scientist. Or a murderer.
Or, hell, what if the person I loved happened to be that guy? I know that at least one of them would have probably said "no." Had someone killed him for his bone marrow, I would have been ... unbearably heartbroken. That isn't a "just" death. To be angry at sickness, at circumstance, is one thing. I am prepared for circumstance. But someone else's actions, with that person and the beneficiary still alive, still happy... no, unbearable.
I know about my circumstances, and I don't know about the circumstances of the other person. I cannot choose for them.
Truth be told, in reality, I'd probably try to introduce my loved one and the callous person to each other--and hope.
Wouldn't you also feel guilty that you could have saved him/her but have chosen not to do so? Would you tell your loved one about your choice or prefer that he/she never knows?
I wouldn't tell my loved one that I didn't murder for them.
That is because I would have never thought to do so.
And no, I honestly don't think I'd feel guilty at all.