Devil's Advocate here, just as food for thought. Let's say that the situation were slightly reversed, and the friend's parents decided to have a "talk" about "morals" with YOUR son. But, let's say that the "moral" that they wanted to push was about something you didn't believe in... perhaps his parents believed that looking at porn is okay and normal. Would you want them having this kind of conversation with YOUR son?
I agree that it makes sense to discuss your personal values with your son and let him know what you expect of him. But, I also agree with the others that it is not your place to do the same with his friend.
The best way you can influence your son and his friend is by providing a positive example yourself, by not being judgemental or implying that they should feel guilty, and by showing with your behavior that you are open to discussing "values" in a way that considers other points of view.
I'm sorry you're going through this, Rhapsody. I know that this is a very sensitive issue for you and I wish that your husband and sons would behave in a way that doesn't trigger you.
Thinking of you, ((( Rhapsody ))),

LMo
(ps - I'm going to be offline for the rest of the morning/afternoon and possibly until Sunday evening, so please don't be upset if you reply and don't hear back from me right away. Thanks)