As for the threats she made today, do you know any of her family or mutual friends who you can make aware/ask to check on her?
BMI isn’t really a great measure of how healthy you are as it doesn’t take into account body fat and muscle weight. Nevertheless, it provides a rough idea of how healthy you are, and there is absolutely no way that 21 is bad. Would you consider yourself physically fit? (I think a good measure at BMI 21 is if you can see any toned muscle on your body or not)