This situation of yours doesn't make any sense.
I'm trying to piece it together like a forensic detective, but keep coming up with the same answer.
You met this girl a while back and you flirted with her, but she had a boyfriend?
How do you know she's currently single?
Where are you getting this information from?
Her friend says her friend doesn't want you.
But you think that's a lie because she hasn't seen you?
You know what I think? I think she has seen you already and is not attracted to you for some reason. (The Boyfriend is still in the picture)
This entire situation happened on Facebook.
You saw someone in your friends network of friends online that you were attracted to, but couldn't figure out how to reach her. She probably has her profile set to private.
This is one giant fantasy buddy!
If her friend is stonewalling you, there's a major reason for that.