Friend is a treasure for your life, then:
Be a good listener for your friend
Do not you ever be patronizing. Giving advice is fine, but do not do it too soon. Do it slowly, but make sure your friend is listening.
Every person has a unique and distinctive personality
Try to understand of how the character belong to your friend. Respect his opinion. Even though you differ in opinions and beliefs, but there must be a middle way as a solution, just do not rush to decide.
Hold the trust that has been given by your friend
Never ever, you unlock the secret of your friend to others. Each keep a secret, suppose that between both of you, there is a game that can only be played by you and your friend.
Give support and praise your friend, leave aside his mistakes and weaknesses
Do not ever...