Good for this guy, cause he has this exactly right. Shaming and humiliation is not a constructive disciplinary technique. It is abusive, and by the way rarely produces long term change. Clearly, sneaking a boy into her room at 3 AM is a HUGE concern, and I'm guessing not the first time. But I am also guessing that if this is how these parents are choosing to punish their daughter, their parenting style in general may be somewhat extreme and authoritarian. Probably not a lot of talking going on in this family.
At some point in your teen's life they will do something extreme that crosses your line. Every family is different, and the standard for crossing the line is different for every family. Responding to this transgression with " How could you do this to me/us?" will get you nowhere. The fact is, your teen did not "do this to you." Your feelings, and wondering what the consequences would be do not even come into play with your teen. For that teen who snuck the boy into her room, what her parents or grandparents would do if she got caught was no where in her horny, impulsive teenage brain. And that is really the issue. Raising a teen means understanding that their egocentricity and narcissism is a part of their personality...for now. It is not a character flaw, but a developmental hurdle that has to be planned for. Your teen acts on emotion and impulse, not thoughtful and careful consideration.
When parents use shame and humiliation as a consequence for this kind of behavior, they are shutting down communication, not opening it up at this very important juncture.
Clearly there need to be consequences for this girl. Both sets of parents need to sit down with the kids and talk not yell about what happened. Rather than banning this boy from the house which might send them out in secret, I would invite him into their house to hang with supervision. I would make sure there was no late night use of cellphones, which is how this 3 AM rendezvous must have been set up. I can imagine a late night sexting/texting communication that ended in a "I need to have sex with you right now" and up the wall into her bedroom he came!
Humiliation and shame can do long term harm to a person, and to their sense of self. Teens are extremely vulnerable as they are in the very beginning stages of assembling their identity. They are just getting the roots in place, and if those roots are stepped on rather than nurtured, there can be grave consequences.