Interested in Language
Alex is a friend of mine.
Alex is a friend of me.
I know the first one is correct but what about the second one?
And while we are on the subject of Mike's suggestion, it does not, despite those who will try to tell you otherwise, suggest there is only one friend. He's my friend as opposed to my cousin, boss, or dentist.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.