There is always a reason why teenagers move in that direction and it is usually because THAT crowd accepts them while the crowd their parents want them to be with for some reason is not as accepting or understanding. The crowd they are moving towards is offering them something they are in need of at the moment. So there is something going on with them right now.
Ask your friend what changes she has seen in her son lately. Ask her what is going on with him lately. Tell her to find out from his teachers, friends and Rebbeim quietly if they noticed any changes or if there is something she should know.