I agree with Pegasus, but having been on the other side of the fence as a teenager, I can definitely say that the worse thing you can do is to allow yourself to get run over. You have to take care of yourself. That does not mean don't be supportive of a depressed person because even rapid mood swings that are caused by environmental factors are real to the person. It can point to several real psychological conditions. The ultimate goal though is for this person to decide to take the reins and get treatment.