This!!

Once she starts eating normal again, she’ll absolutely gain the weight back. In fact, she’ll likely gain back extra weight because her body will be in starvation mode and it won’t burn as many calories.

u/nana_banana2 NTA and you’re right to be concerned about your friend. I work with people with EDs, and nearly every single one of them started out on an absurdly restrictive diet. And EDs have the highest mortality rate of any mental illness. So no. You are absolutely NTA.

Honestly though, I wouldn’t mention it again. Your friend knows that 800 calories a day isn’t enough. She knows it isn’t healthy. If she says otherwise, she’s either deluding herself or she’s decided that the risk of developing a life-threatening ED is worth losing some weight. Either way, nothing you say will change her mind. In fact, it might make her dig her heels in more in order to reduce the cognitive dissonance. The best thing you can do at this point is to provide your friend emotional support (no practical advice) and make sure that you’re a “safe person” to talk to if (and when) her diet starts having negative consequences.