* Γεια σου, φίλε μου (Gia sou, file mou): This is a common and informal way to say "Hello, my friend," used for a male friend. Γεια σου (Gia sou) is a general informal greeting. Φίλε (file) means friend, and μου (mou) is the possessive pronoun "my."
* Γεια σου, φίλη μου (Gia sou, fili mou): This is the equivalent for a female friend. Φίλη (fili) is the feminine form of "friend."
* Χαίρετε, φίλε μου (Chairete, file mou): This is a slightly more formal way to say "Hello, my friend" to a male friend. Χαίρετε (Chairete) is a more formal greeting, plural but often used as a singular polite greeting.
* Χαίρετε, φίλη μου (Chairete, fili mou): This is the more formal equivalent for a female friend.
* Καλημέρα, φίλε μου / Καλημέρα, φίλη μου (Kalimera, file mou / Kalimera, fili mou): This translates to "Good morning, my friend," and is appropriate for the morning. You would use "file mou" for a male friend and "fili mou" for a female friend. Similarly you can use καλησπέρα (kalispera) for "Good evening/afternoon"
For most situations, Γεια σου, φίλε μου (for male friends) or Γεια σου, φίλη μου (for female friends) will be perfectly appropriate and friendly. If you are unsure of the gender, or want to be more formal, Χαίρετε, φίλε/φίλη μου is a safe option (although slightly less common in casual conversation).