Friend/weed problems

My friend (16) has been smoking weed for the last couple weeks, and he's been trying to make me (16) join in ever since. Now I have no desire to smoke weed. I'm pretty neutral on the subject but I just don't want to. But here's where it gets difficult. He also has severe depression, and he has started saying that if I smoke with him it will help his depression. I'm not sure if he is sincere or trying to guilt trip me. But regardless it has put me in a tough spot. So what should I do? All advice is welcome.