Is it possible to "recover" from an eating disorder without gaining weight if you're at a healthy weight?

My w/o buddy are on reduced calorie diets. What she failed to tell me is that she had/has an eating disorder. I did as well but she knew about it. I'm trying to convince her that she can increase her intake without gaining weight.

We both hit a plateau a while back that we're both trying to bust by increasing protein intake and working out more and doing different workouts with strength training. I'm losing slowly and my intake is the min recommended by my old RD. She's losing very slowly too BUT she's eating no more than 400 cals a day while working out pretty hard almost every day. I saw her food and exercise log. Our BMIs are 21/22.
She gained two dress sizes (~20 lbs, 24 or so BMI) when she got help before with "restricting."
I love working out with her but I don't want to go back to that again and I know I will. When we hung out last wkend, I ate exactly what she did that day, 300 cals max.
How do I get her to up intake without gaining? Can't find anything online

If she's only eating 400 calories a day, she still has an eating disorder. In order to lose weight and gain muscle the body needs to burn calories. If she doesn't consume more, her body will go into starvation mode (probably already has) and start to turn her existing muscle into fat so it can be burned for daily processes). Fat burning and muscle building/toning are processes for which the body requires calories to carry out. They can't happen if enough calories are not being consumed.

Before she can begin losing weight, she needs to deal with her eating disorder and get herself eating healthily. This kind of restriction is going to do her more harm than good in the long run.