A person of African origin may enjoy a highly spiced diet, and the smells from some of these spices extrude through the skin pores in the following days.
Thus, the odour is not BO as such - that is caused by the bacteria that breed in sltale perspiration on the skin and in clothing.
It may be that the lady has a perfectly adequate hygene routine, but that her diet may be the issue. If you approach the issue from that perspective you may be more successful, because the problem is caused not by sufficient cleanliness, but simply by eating pungent foofstuffs.
You could always bring the conversation round to garlic, and its lingering after-odour, and then start to talk about her diet, and mention that the same effect happens for 'some people'. If she doesn;t work out your hints from there, you may have to simply tell her, as nicely as possible.
Personally, I'd rather know, rather than whiffing away in blissful ignorance - it's the nicer of the two evils.