As a teenager, I had an experience with my friend where we snuck out of her house and ended up at her boyfriend's house. That night, I made a lot of stupid decisions, but ultimatly, I pretended to be asleep on the floor while she and her boyfriend had sex. I was 14. I did not have anyone whom I felt comfortable talking to about the situation, so I never did. On one hand, it is good that your daughter feels comfortable discussing such touchy topics around a someone else, especially a man.
Here is the problem though. If you confront her, you may be making your boyfriend the enemy in this situation. She trusts him, and will feel betrayed if she thinks you are using him to spy on her.
If you are really uncomfortable with what happened, talk to your boyfriend. This way he knows how you feel about the situation and in the future he will put a stop to the converstation. You can also encourage him to talk to your daughter. If he is your boyfriend, then he must be some sort of parental figure in her life, and may be able to talk to her about what is appropriate and not appropriate.
My husband has a younger sister who is almost 19. A few years ago, a 20 something guy was living in her home, and we walked in one day and found her sitting on his lap. I was mortified. I let my husband "talk" to the guy while I talked with her. She was young but she still should have known better. Your daughter is old enough to know where the line is as well, but if you do decide to talk to her about it, keep it simple (Hey, Joe was uncomfortable with the topic in the car the other day...you may want to tell your friend to keep the details of her sex life limited to when you two are alone) and leave it at that.
One final thought...Is this really a mountain, or are you just shocked that your daughter may be having sex? Shoot, I would be!