Dude, while I agree that long-term marijuana use may have long-term effects, your PhD friend may still be flawed in his interpretation. I've heard this very thing, exactly this in fact, from plenty of anti-cannabis proponents. But the fact is, the methodology is deeply flawed. As you said yourself, there are tonnes of kids coming in with mental problems, particularily depression, and they smoke marijuana. The interpretive flaw here is that they're basically saying "these kids are depressed, these kids smoke marijuana, therefor marijuana made them depressed."

They're linking 2 common characteristics then creating the association before even validating the theory. Using marijuana and being depressed can mean marijuana made you depressed... but for how many? What about kids who are depressed in the first place, and thus use marijuana to alleviate their symptoms? What about mentally ill kids who smoke it to feel better? Most people who develope mental illnesses later in life have precurser psychological conditions that are not yet strong enough to be given a clinical title. It's only logical that anybody with some sort of psychological issue is going to be under chronic stress, and thus grasp on to something that alleviates this stress.

Scientists may know a lot of facts, and they may know more about biological mechanisms than most of us, but everybody is subject to logical flaws in their reasoning. It's this very kind of backward reasoning that is finally seeing a backlash from a society fed up with being lied to.