The Cobalt will grey out the pressure display when a gas switch is made away from the primary mix- but gas switching needs to be enabled beforehand. If a switch was made it will show in the log. Otherwise it's hard to see how just a portion of the display could fade out.
Is it possible that ambient light washing out the display had something to do with it? Gas pressure does display in a different color, maybe that was hard to see under the lighting conditions? Was the pressure display truly not there, or just not readable? If there was something odd going on that caused the pressure display to go away (not something we have ever heard of happening), it would help to know the firmware version she was using. If it has worked on subsequent dives, and the display appears normal, no splotches or damaged looking areas, I don't suspect a problem with the hardware.
Ron