Meaning of the slang Boo
That slang term for Boo is often used as a term for a Boyfriend or Girlfriend or even towards a friend in the right setting. Think of it as a pet name similar to how you would use hunny/honey or the currently popular Bae.
John: "Hey Dave!"
Dave: "Hey Boo!"
Whilst typically it is reserved for a more romantic relationship it can be said between friends in a partially mocking/endeering way the same way you may refer to a friend as "hun" or "honey".
Urban Dictionary : Boo