I think you missed Macdonald's point. He's saying that "real life" is not simply the continuance of conscious existence, but "human thriving". If the man could achieve human thriving without health and without friends, then he'd be fine; it's precisely because he cannot achieve "human thriving" without health and without friends that that he says "I'm tired of life". Health problems, pain, and loneliness aren't life, but a form of death. So, MacDonald says, he's not tired of life -- he's tired of the continuance of conscious existence without life; he is, in fact, tired of death.
"Adults aged 75 and older have one of the highest suicide rates (20.3 per 100,000). Men aged 75 and older have the highest rate (42.2 per 100,000) compared to other age groups." [1]
The old man needs health and life. Without health, life would just be a tortured existence and at some point "enough is enough".*
* It doesn't help that for many old people, all their friends are also gone. Without their social circle, it's one less reason to hang around.