The metaphors with "money in bank" or a "non-working car" are not applicable to this case because the miles expire. If you wake up tomorrow the car will remain in the garage, money will be in the bank, but the airlines will take the miles away. So by not acting in this case, you are only helping the airline reduce their liability.

That said, what somebody does with his miles is his business. You can reason with the friend to give it to you for something in exchange,or give it to charity or buy magazine subscriptions to your local school or library. Taking his miles without his consent is pure and simple thievery.