What happens to people who owe money to drug dealers?

I don’t owe money, but I know someone who does and I am extremely worried about them. I don’t know how much they owe, but I am scared they will end up dead. I guess this is more two questions in one but I want to keep the title question short.
1) what happens to people who owe money to drug dealers?
2) What do you do if you know someone (a minor) who owes money to a drug dealer?
This person is making a lot of illegal choices such as the age of the person she is dating and with owing money. She is my best friend and I’m very worried about her. She is also going to be starting school in a city that is notorious for gangs, drugs, and violence so that heightens my worries. Your advice is greatly appreciated, I am very lost.