I'm surprised everybody's been so opposed to you so far. Even before I read you were together 24/7 I agreed with you. If anyone I cared about was being really unhealthy I would try and talk to them, and I'd hope if I were doing the same thing someone would tell me.
As someone who has tried to lose weight their entire lives I've done A LOT of research on the subject. 1200 calories is by far the lowest anyone should ever go as a long term life change. Even that is really too low to be sustainable for the rest of your life, especially when you are exercising.
You've been really successful in your weight loss so I'd think your friend would look up to you in this situation and listen to your opinion. I know if someone told me I was eating or doing anything unhealthy, when they told me I'd probably be angry with them but it would get me to think. So even if your friend doesn't seem to take your advice she's thinking about it and it probably will make her double think her choices especially since it's coming from someone she loves and you've been successful.
Good luck, I know it's not the easiest situation.