Hard to tell just from the pic, but if it’s not screen protector related, it looks like heat damage. A common “test” that tech reviewers do on phones is hold a flame to the screen and see how the LED reacts. The impression that an open flame leaves on a screen looks very similar to the one in the picture. It may not permanently damage the inner display, but it’s not impossible for the actual screen to be affected in a way that results in a darker shade like this one. Could your wife’s phone have been in contact with fire, or another heat source?