There are some individuals who get physically addicted (it sounds like your friend is one) and a small number of people who are vulnerable to mental problems like schizophrenia can set the illness off by using marijuana. That's about 1 person in 100. Pot can trigger an episode years earlier than it normally starts. That's very rare, though. So, the younger you start using, the more problems you are going to have from using pot, but your friend is not going to die because they use marijuana.

The more common mental problems with pot (poor memory, bad concentration, loss of drive and motivation) should all get better when your friend stops using. The longer they go on using, the longer recovery takes, but it is rare for any permanent damage to occur. Heavy use in the teens can also lead to long periiods of depression and anxiety later in life but again, this gets better when people stop using.

Probably the best thing you can do is pass the problem over to a trusted adult. Some school teachers are good, and the local health centre may be able to give you some support too. Look them up in the phone book and have a chat. You don't need to give your name.