This subject can be as open ended as "what table saw should I get" 🙂

Typically I'd say see what available body are still compatible with her lenses.

Much of the improvements in bodies over the last decade have been in the image processing side versus the actual light collection side.

Cameras can focus faster and in lower light conditions. Processors can do a better faster job at moving the image from the sensor to the memory card.

Do you need to take pictures in low light or need rapid focus for quick shots? Improvements in either of these areas entail higher costs, if not so important then say the money for where you need the boost.

Actual improvements in the sensors has been less noteworthy unless you are always seeking the very lowest noise and highest sensitivity possible.

"Mega pixels" is really a useless term, are they counting a pixel as something that can capture red/green/blue or as a single photon bucket that captures just one color. The key parameter is the size of the sensor, larger means more silicone dedicated to receiving light.