hi..thanks for your query..
From the clinical picture described, I feel your wife has something called Melasma.also known as Chloasma
faciei,is a dark skin discoloration. Although it can affect anyone, melasma is particularly common in women, especially pregnant women and those who are taking oral or patch contraceptives or hormone replacement therapy
(HRT) medications.The symptoms of melasma are dark, irregular well demarcated hyper pigmented patches commonly found on the upper cheek, nose, upper lip
and forehead. Melasma does not cause any other symptoms beyond the cosmetic discoloration.Treatments to hasten the fading of the discolored patches include:
Topical depigmenting agents, such as hydroquinone
It is extremely essential to use a broad spectrum sunscreen throughout the day , with repetitions every 3-4 hourly,not only when outside, but also even indoor or while cooking.
Hope this helps..Take care!