Hi, sweetheart. Yeah, you are right. It's just ' a friend of mine' is more frequently used in the everyday conversation, even when everyone knows you are referring to YOUR OWN friend. I guess that's why language learing can be annoying sometimes, because, it's not about reasoning and logic; its more about norms and conventions.
What I mean is that "recommend" is "recommend to do something, recommend someone to do something, or recommend something to someone". But can't recommend someone to someone.
For instance a waiter recommended a meal or drink to customer.