You sound a great mum and sorry to hear about your daughter :(. This J person sounds really toxic and the issue seems to lie with her. The more your daughter hangs around this person, the more her health and self confidence will be affected longer term. Harsh as it sounds, it is best to sit her down to see how she can remove J from her life completely. I had to do that with an ex-best friend I had for over 20 years after the relationship turned sour and affected my mental and physical health. I had to stop all contact with them, even though we lived in the same village. It was the best thing I've ever done as I have not only grown stronger as a person, I've let in people who truly inspire me and are just amazing people. :)