Also I doubt that exercising will make her feel better at her state right now, it isn't good for her mental state at her current physique. I'm not attacking you or anything aha just expressing my opinion on this
Hmm. I see where you're coming from but I think exercise, in healthy amounts, will be just right for her mental state right now. For starters, it would result in healthier weight gain. I'd argue that putting on weight in a healthy way will leave a person "better looking", and thus possibly more confident and happier about that weight gain, than if they were to gain weight by sitting around doing nothing and eating lots. It could also help in establishing good and healthy habits. I know it can also be unhelpful and recovery is very complex either way, but yeah... either way, it certainly is not an easy journey. And I can see your concern about exercise at low weights. However, Xacatecas isn't advocating hardcore sessions, from what I can gauge. And gaining just fat (or just muscle) is impossible, anyway. It'll always be some sort of combination unless a person is extremely sedentary, which would be incredibly unhealthy. Sitting for prolonged periods is linked to lots of health complications.
Anyway, OP - obviously ultimately it's your choice... whatever you do, stay safe!