Neither of my parents are here any more, but I'm not able to answer this particular question. I just don't know. As a parent, I wouldn't be willing to accept one of my children's lungs (or anything else); so I'm assuming (fairly certain, in fact) my parents would have felt the same way.
Both of my parents would have said, "I've had my life, and this is fine. I'm OK with it. Let me go, and you go on and have your own life - and don't even think about this."