How to tell your boss she has B.O?

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I have a new boss reported duty last week. She is single and very nice. Except that she has Body Odor! Before she came to the workplace, someone already told me er problem. She rarely take bath and never shaved her armpit. I did not believe it until I see and smell it myself! Her hair was like tangled wire and when she lifts her arms, Oh My God! I can smell it like in a few foot away! I can tell that other people can smell it too because there are covering their noses but nobody has the courage to tell that to her. I've had this problem before with my staff. But I can talk to her about her problem because she is younger than me. But my boss is about 50 years old and very superior of me. So ho do I tell her about her BO? Should I tell her at all?