Advice please for a friend!

I do have a serious problem though. One of my friends who has been losing weight for two years now (lost 70 lbs so far, is around 180lbs now) is starting to adopt anorexic behavior. Technically, she isn’t developing anorexia, but ednos since she is not classified as underweight. I was extremely worried so I had a serious talk with her and even pulled out information from an eating disorder help website to back up my worries since she was in denial. I asked her what she’s been eating and it’s only been adding up to around 500 calories! AND this girl is in marching band and insists on extra work outs. I’m incredibly worried about her physical and mental health. She states that she thinks she’s fat and nothing else works and she just wants to be skinny (I adamantly refuted this, she is not fat and skinny isn’t happiness.)
Please tell me what you guys think. Should I wait a few more weeks and if this behavior persists should I seek an adults advice? I want to help before the damage becomes long term, but what if this behavior merely turns into a phase?

First and foremost, thank you so much to all my new followers and the really sweet encouragement! It made my day. That aside, down to 160.8lbs. So close to the 150lbs! I haven’t really had time to go to the gym this week except for a couple of times, but this is totally fine for me since I march two hours a day anyways. Still lifting heavy, especially barbell squats since they’re my fave. do you guys have a favorite weight exercise?