Although my husband and I were blood matches, a good start for living donor kidney transplant, we were both turned down when we tried to donate to our friend who was going on the list. I previously had breast cancer and was turned down for that reason. My husband is 79 and has early Alzheimer's although he can tell you why he wants to donate and understands the process. He was turned down because of age and the impact of major surgery on dementia.
What we have done is to stay in contact with our friend, to share the process and outcome with him and his wife so that they know what donors need to be doing, and we also gave them a bear totem, a Native American symbol of strength and healing which he finds comforting.