Generally, there are two factors to consider: legal and ethical. They don't always converge. However, in a simple situation like this, where you are asking if a song can be shared with a friend, I would consider this: instead of a digital copy, if you had a CD, sharing with a friend or family member is perfectly legal. Hence, you will be within accepted legal parameters in sharing your song privately with one friend. You break the law when you make it widely available.