Dell G2410, LED backlit Display

CRTs do really flicker, because there is constantly beam "illuminating" the pixels, which writes each horizontal line from the top to the bottom of the image. And that can flicker at 60Hz. And that can be felt by the eye.

LCD displays are made up of many layers of transparent materials, an LCD panel and a backlight.
The LCD panel is a panel with lots of dots (pixeles) which vary their light transmition (from black-opaque to transparent). You can set its refresh rate from the computer, but normally you have to use a lower resolution than the maximum to be able to increase the refresh rate. Anyway, I don't think this is related to her problem, because the LCDs don't turn on and off, they just change the intensity (opaque, transparent, or in between) of the pixels. A 60Hz refresh rate means that the display is refreshing its information 60 times per second.

The backlight is a big white square light. Standard LCDs are lit by a compact fluorescent light (CCFL). Newer ones are lit by LED lighting.
Fluorescents need to flicker in order to work. There is no way to make them work without flickering. A magnetic ballast (like a brick inside home fluorescents fixtures) works at 60Hz (this means 60 times per second). An electronic ballast works at a much higher rate, I'm not sure but it might be around 20KHz (20 thousand times per second).
Very sensitive eyes can notice the difference between a 60Hz tube and a higher one. Even if you don't notice the difference, it is evident that after some hours of being inside a place lighted by magnetic-ballast-tubes, the sight gets more tired than with an electronic one.
I don't know what is her "hertz limit", but according to what the eye can normally perceive, no one should be ever affected by a 20KHz flickering tube.
My display shows 64.5KHz on the menu. That's the backlight frequency.

LEDs don't need to turn on and off to work. You can connect a LED light to a DC battery and it will work. But to be able to dim a LED, it's better to use a PWM dimmer instead of a standard dimmer. This means that instead of decreasing the voltage (which increases the temperature of the LED and reduces the lifetime), you turn it on and off, and you change the intensity by modifying the on-off time. Imagine the cycle lasts 1millisecond, then the LED can be 0,1ms on and 0,9ms off, or 0,9ms on and 0,9ms off, or in between.
LED baclkit LCDs, as any product with a dimming led, use this system. I made a simple PWM dimmer for my LEDs at home and when testing it, it shows to work at 20-22KHz. I don't know which is the frequency of the LED monitors, but I can say that it cannot be near a noticeable number (for the eye).

The only differences I can think of between LED backlit LCDs and CCFL backlit ones are:
- LEDs (as they use RGB LEDs) produce higher color gamut, better color reproduction.
- LEDs don't contain mercury, nicer to the environment.
- LEDs have a lower power consumption. But I'm not sure if LED monitors do consume less power, because they are brighter, which makes them be the same wattage compared to CCFL LCDs.

I think this woman, or maybe you, should ask an ophthalmologist about which is better.

Regards,
Rodrigo.

LED monitors have a built in PWM dimmer - that's what he was saying. It will flicker, as will fluorescent lit LCDs, but the flicker frequency is so high that it is unnoticeable.