Has she always been thin or has she dropped in weight recently? Look out for signs of skipping meals and not eating in front of others. Maybe you could suggest going out for food and seeing how she reacts? When I had my ED I would have avoided eating socially or thrown up after eating. It's good that you're looking out for your friend
I am slim (7.5 stone) and definitely have some "issues" with food - when im stressed with work etc I seem to burn through everything i eat and that's when i remember to eat at all. My friends are pretty direct about it and telling me they are worried about my health which doesn't bother me. I mean it doesn't change anything, i already know my issues and i am working on them at my own pace but it doesn't offend me that they mention it - I know it comes from a good place. If your friend is under 6 stone however, that sounds a lot more serious and like she would urgently need help. How long has she been this weight? Is she unwell? Because if she isn't then I would imagine she would get overall very poorly very soon at that weight
Sorry meant to add - I don't think there is anything you can do except tell her truthfully that you are worried about her - it is difficult to know how people will react but it is also important to know that people are concerned and people notice.
I have a friend with anorexia - I don't know how she survives she is a walking skeleton. She has been sectioned a few times. She has been obviously Ill for around 10 years, looking back she has had issues with food for a long time. She also has OCD amongst the mental issues and other physical problems relating to the over excersising /lack of nutrition. She seems trapped in herself, she is a very loving/giving/ she knows she has a problem. She avoids the availble help. It's sad for the people around her but you can't make people take help.