What a hard position you are in. I wonder if the best thing you can do is just be straightfoward, but gentle in telling her what your concerns are. Tell her how much you value the closeness of your relationship and that you are not trying to be critical at all - instead you are acting out of maternal care. I don't think there is a way around this being difficult, but I do think if you are gentle and straightforward in stating your concerns, you will be treating her like the adult she is. Also, if she starts arguing with you about what you say, don't argue back. Just share your perception of things.
Probably haven't said a thing that you don't already know. It is a good thing that you are close, as she must trust you.