I agree with your thoughts. I feel if the friend or her family knew of someone else that DS was going to see first 'before' going to her house, and THEN she still didn't show, that would have raised more of a concern with them. Maybe thinking, "Well, she said she was going to do XXXX first/meet with XXXX/stop at XXXXX...and that's weird that her phone is off and she still isn't here, maybe I should reach out to someone..." or whatever.
Otherwise, I think most of us would just brush it off and wait to contact someone later/next day. We don't know what the friend knew/said/did, except for what was released, which is not much. I find it hard to believe that most of us would be calling the police if a planned night/event fell through. Don't get me wrong, the more I follow cases, I begin to wonder about that stuff, but calling the police to say "my friend was supposed to come over but she isn't here?" Maybe drive over to their house if I was really concerned or contact a family/friend, but still feel I would probably be told "everything is fine, don't worry" until a day went by, like in this case.