Formal:
* ¿Cómo está usted, mi amigo? (This is very formal and would be used for someone you don't know well or are showing respect to.)
* ¿Cómo está usted, amigo mío? (This is slightly less formal than the first option.)
Informal:
* ¿Cómo estás, amigo? (This is the most common and informal way to say "how are you, my friend".)
* ¿Qué tal, amigo? (This is a more casual way to say "how are you" and can be used with close friends.)
* ¿Cómo andas, amigo? (This is a very colloquial way to say "how are you" and is often used with close friends.)
You can also choose to use a different word instead of "amigo" depending on the relationship:
* hermano/a (brother/sister) for a close friend
* compañero/a (companion) for a friend you work or study with
* querido/a (dear) for a friend you have a lot of affection for
Example:
* ¿Cómo estás, querido amigo? (How are you, dear friend?)
It's important to note that the most appropriate way to say "how are you, my friend" depends on the context and your relationship with the person you are talking to.