Teenagers have to recognize if their friend needs serious help because it can lead to very bad situations, if they're not treated properly. Warning signs may be distancing themselves from others, not talking a lot, getting into situations, they wouldn't want to get in before, not being in the same mood as they would always be, and even the most obvious ones where they explain their mental condition. You have to know who to turn to, if you can't provide the help that your friend needs, you can advise them to go to a therapist, call a hotline, speak with their loved ones, but you have to know that you can't brush it off, or say that "it's a phase", " stop being dramatic ", " you're too young to have these types of problems" etc. such phrases can be discouraging towards anyone and can lead to even worse situations.