Not eating is no way to lose weight. Sure, it will have an initial impact but a lot of what you lose is in fluid in the first instance. It's not sustainable in the long term, if she starts eating normally again the weight will come back. If she continues to avoid eating then it will take a toll on her body. The longer she continues, the more of a toll it will take. The road to anorexia is paved with unhealthy skin, hair that falls out, fingernails that are weak and brittle, an inability to go to the toilet, a cessation of periods in females and onwards to gradual organ failure. There is nothing remotely glamorous about that and it doesn't take long to get so deep in bad habits that you can't break them.
Losing weight by avoiding food is an illusion, it's not an achievement. If someone wants to lose weight and be healthy then the only answer is a balanced diet and a regime of exercise. I think your friend needs to realise where not eating leads. We don't eat for fun, we eat because we need to. Our body relies on that energy in order to survive. If you threaten that survival then the body starts to shut down all non vital functions one by one in order to survive.
I hope your friend sees sense soon.