If you knew she was someones girlfriend why is she in your bedroom. How would you feel if it was your girlfriend
Tell her that you made a mistake and Let know know this shouldn't happen.
Tell her that you love her
show her your penis...
Go in the room with her, that should do it.
Sorry dude, you have to keep her!
i can help u,just say mum get out i need you to respect my space looool