I definitely do NOT advise commenting on her weight or size as a way to open things. You might open up some dialogues about health in general, i.e. exercises you like to do, eating practices you like to keep, vitamins you take, health goals of your own, as a way to hear what she has to say on these topics. BUT there are a million and one reasons a person might have a dramatic weight loss, and anorexia nervosa is not the most common reason. You are a wonderful friend to be concerned, just be sure you don't jump to conclusions.
A word on so-called "pro-ana" sites - you can no more GIVE someone anorexia nervosa than you could GIVE someone schizophrenia. So I don't believe they truly make anyone mentally ill. They can however, give someone enough bad diet and health "advice" to make them physically ill if the person follows that advice. I don't wish them to be illegal because I believe in free speech, free expression, and free exchange of ideas. (Even if the ideas are bad) but I DO think they are in poor taste. A great number of them have actually been removed in recent years.
I hope your friend will be able to open up to you about whatever it is that's going on. Whatever it may be, I am sure she will be very happy to have a caring, non-judgemental, supportive friend at her side.