Most Helpful Guy
This happens. Homegirl lied to you in order to spare your feelings ( but really not to make her look like a bad guy for rejecting a potentially good guy). She may think that you couldn't handle the truth aka patronizing you.
If the right guy/girl comes around NOTHING will stop people from allowing something to happen. I'm sure that if Ryan Gosling or Ian Somerholder came to a girl at her worst and asked her out I know many who would say yes and work things out desppite their prior problems.
Your girl was single and available, just not for you. When you confronted her she had to continue her lies/story