I'm not sure I can directly answer your questions, but I can provide some insight and experience. I'm 29, fully independent, and survived having a narcissistic mother. But, it wasn't without a price. My relationship with my siblings is damaged as a result.
Hindsight, I wished that I was the oldest (as opposed to the youngest) and that I implemented a strategy where my younger siblings and I spent lots of time together and had open communication with each other about everything including how they felt about our mother's behavior. Our ages were a little spread out. My sister, two years older than I. My brother, 7 years older than I. Growing up, I watched as my brother and sister had constant battles with my mother. The tough battles between my mother and I didn't start until I was older and actually moved out of the house. So it was easier for me to endure them since I wasn't under her roof anymore. But I never had any help from my older siblings in dealing with her. They were very selfish about it. Hindsight, I think my mother actually turned me against my siblings. She would constantly talk bad about them to me. And it would give me a bad impression of them.
Make sure that this doesn't happen to your friend. He will be MUCH better off if he's on good terms with his siblings in 10-20 years. He needs to band together with his siblings and it has to start now. They should always be in the top 5 most important people in his life and he should treat them that way. He's the oldest so he needs to lead by example and take charge. Ask them how they feel about the way their mother treats them. Tell them he cares. Give them a hug.
Try to pick and choose your fights wisely. Don't always go looking to pick a fight with your mother every selfish comment or action.
Becoming 100% independent is critical. Especially financially. Being in his mid-teens, your friend is at the point of his life where he's making some very life decisions regarding his career. If his parents are helping him financially by paying for his college, then take it. Get an education for a degree that is proven to provide a good job. If his parents aren't planning on paying for his education, then he's going to have to work hard for scholarships. If he can't get any scholarships, then he'll need to either go straight into working or try working full/part time + community college part time. If he's into software and his parents aren't helping him financially, he could try working full time + contributing on github. Learn python. If he likes it, become an expert with it. Make projects and put them on github. Within 2-5 years he'll have himself an impressive resume and could get a job literally using that Python experience.
I'm not sure how he can heal. I'll be honest, I still haven't fully healed. You just learn to accept your mother for the way she is. But acceptance doesn't mean you let her treat you poorly (but again, do pick your battles).