Face, neck & hands dark skin aftr pregnc

Hi. My sister delivered a healthy baby and during her pregnancy her facial, neck and hands skin color turned dark. We thought it cud be due to pregnancy. But now its almost an year and it is still the same. She applied home remedies, facepacks and lot of things but to no avail. It looks totally like burnt and black. We recently met a dermatologist and he suggested a gel cutiyt g. He mentioned she must have gained weight but during her pregnancy she didn't gain weight much and after delivery as well, she was never overweight. I would request if someone could suggest me some cure to this. As she has gradually started feeling low and awkward about this. She avoids going to public places and functions because ladies around her starts questioning her. Please help me with a genuine solution to this problem. Thanks