A person should have the final say over their body ultimately. If a person decided that their child's life is m ore important than their own, and their child needs a heart to survive but does not qualify for a donor, then the parent should have the right to be able to sacrifice themselves to keep their child alive.
A person may decide that the life of a friend or family member is worth more than their own and that is completely legitimate. However the person should be assessed by a psychologist to make sure they are not doing it because of suicidality or depression, that it is out of a genuine desire to save the other person's life.
If a person chooses to put their life at risk in order to save another person, then we shouldn't stop them from doing so. If it is something that is being forced, then I would say no. Let a person give their life nobly if they choose that path for their life.
Organ donations are meant to save lives. This is their whole purpose. To kill a donor to save another person's life goes against this very idea. Why should we take a life to save a life? No life is more precious than another and that is why an organ donation should not be done at the expense of another person's life.