Weeelll l if your feeling underhanded may I suggest creating a backup plan. If you fail to retrieve the evidence before you get caught, make damn sure you can sell the, "it was HER fault arguement"

no matter what stick to this story, even when faced with irrefutable evidence, the other girl got you drunk after your ex had gone to bed, you were the innocent here, remeber she kissed you, she led you to the laundry room

If anything the ex should be mad at her friend, why did she **** you anyway what kind of friend is she etc etc etc
Its your best chance of remaining friends with the ex, just dont expect the friend to like you too much.