Best thing to do (since some parents dont bother to call and let you know what's going on) in the future is to ask your son who all would be spending the night and let him know that you would be calling the parents to get the details. Since his girlfriend was there he probably knew about it though I'm not blaming him at all just another way for you to cover everything.
When my oldest daughter was 17 she use to want to go to parties, I would always tell her to give me the number so I could call and make sure the parents were going to be there, funny thing most of the time I never received the phone and she would change her mind about going, LOL, like I didn't know why.
When my oldest daughter was 17 she use to want to go to parties, I would always tell her to give me the number so I could call and make sure the parents were going to be there, funny thing most of the time I never received the phone and she would change her mind about going, LOL, like I didn't know why.