I'm curious why you think something should be done about this? Many women--and men--have darker areas on their skin, even in/on their buttocks and genital areas.
This is completely normal, very common, and nothing to be concerned about.
Some individuals feel the need to bleach these delicate areas of the skin, which can lead to very serious side effects (skin burning, for one), and is seen primarily in women and men who work in the sex industry.
I hope you love your wife's skin...pale and darkness and blemishes and all!