I'd tell her "If you want a relationship with me, we can discuss that...but if all you want is to cheat on your boyfriend, go find someone else.".

__________________
Though I am different from you,
We were born involved in one another.

For it was not into my ear you whispered, but into my heart. It was not my lips you kissed, but my soul.

Complete surrender should not just come at moments in which one faces overwhelming odds, but in the calm when it seems one is personally in complete control of one's life.