nachas:
This is what Rabbi Finkel here in Chicago reccomends in these situations. If the parents are ready to listen and make the neccesary adjustments, the parents should invite him to a discussion. Ask the following question, “We are concerned that you are good friends with a crowd that makes poor decisions. This is not something we advocated and brought you up with. We are all ears, please explain to us where did we go wrong?”
If there was ever a relationship between the parents and teen, the teen’s verbal floodgates will open. Don’t get emotional, defensive, argue, tell him/her that they are stupid, etc., just listen and ask how the teen would reccomend to strengthen the parent-child relationship.
From testimony of quite a few parents, this is sage advice. Giving speeches how much you love him/her and your door is always open, etc. are meaningless parenting class lines. Doing the above really shows that you love and care about the teen.
Bracha Vehatzlachah to your friend and all those who go through this tzaar gidul bonim. Think about the abocve recommendation and let us hear how it went.