If IΒ were her and she were me, perhaps nothing would be different about that time the two of us met. We would each assume with a touch of pity that the other was adorably naive in her opinion of you and her together.
If I were her and she were me, she would find three strands of my hair tangled in your sheets and her chest would sting with regret as she hashed and rehashed every imagined detail that began to crystallize.
If I were her and she were me, she would not be able to look at you for very long at all without the consuming thought of you looking at me (in an identical or different fashion) bleeding in.
If I were her and she were me, she would never touch the subject, never approach it, never cross it; instead, she would let her heart fill up with you anyway, and I would be smart.