Oh, shit! I'm loving this girl more and more!
He said he couldn't go out with the girl that confessed to him because those feelings would be based on a lie, yet his relationship with his best friend was based on a lie too. Logic would dictate they can't be friends anymore because of that.
Obviously she's pushing him out of her life so he'll be able to grow out of that childish judgement, and she's willing to let go of her best friend so he can be happy with a girl she knows will be there by his side when he needs it. This girl is the best wingwoman a wimpy main character could ask for!