I trust that Koko can figure out how to bait him in a subtle way, as opposed to being hostile towards him. Maybe she introduces the topic of the movie she just rented, which just so happens to be "A Beautiful Life" or "Sophie's Choice" or "Schindler's List". Or, she brings up a book she just read like "The Nazi Officer's Life" or "Let me go". Or, she brings up something about her relative's recent Bar Mitzvah or, simply, the message she heard last week at Temple.
She can ask everyone what they think Obama's chances are in the upcoming presidential election.
And so on and so forth using the water drop approach rather than a "bat to the head" approach. If Koko is very pleasant and cordial during the discussions, he'll look like a jerk when he starts to crack and she'll be in the clear. It won't take too long; a 17 year old buy doesn't have the stamina to maintain a "false front" for very long. Let's hope Diana isn't so enthralled with being on a date that she misses the red flag when it waves in front of her face.