It is not to prove to a friend. It is to prove not only to Shaitaan that Humanity will act in accordance with Allah, but to prove it to Humanity itself. A person can be born with great obstacles yes. But with those obstacles there is always room for optimism and faith. Faith that Allah will reward you struggles infinitely beyond comprehension. Yes, the person can choose to turn away from Allah, that would be his choice. But, you hint that Allah would punish them for it. Do you know what Allah is thinking, how he ascribes punishment? Allah does not deal in absolutes he looks at every person individually and knows all, it is a case by case basis. Will Allah punish the person for turning away? Perhaps, perhaps not, what one does know is that the best way to evoke the sympathy of Allah is to act in accordance with the life of a Muslim.