Certainly not a relative or friend. The person most qualified to inform the patient and to manage the case is the doctor who has the patient’s trust. Trust is essential.
The main reason why the case has to be handled by the doctor is that this assigns the proper roles to the various persons involved. The doctor will handle the technical decisions and explanations; the family will provide love, support and understanding. The patient needs help from both sides.
Moreover, in this way, the doctor also acts as a kind of “lightning conductor“, leaving family members free to play their most natural role.
At the same time, in addition to the technical decisions that the doctor has to take, there is also the delicate question of how to inform the patient. Family members, who have no technical expertise, cannot know what to say and how to say it, and, especially, when to stop.