assuming we're talking about adults, I really don't see how you could succeed.. let's be honest..unless she'g to a bad case of the STDs, most men are willing to take the quest, if the reward is a girl with many levels in nookie. You could warn them in a "look..this is what happened to 4 of my friends already"..way but that's where it stops, because anything more than that might jeopardize your friendship with them. (unless she's very flexible and you have a large suitcase..in which case you can send her to me)
also.. tell them that if it goes wrong, they owe you a beer down at the pub.
at least you'll be drinking for free.