Replacing A/C thermostat
Thermostats are pretty universal, and they are really just a set of programmable contact closures at their heart.  Call for A/C?  Connect the red and yellow and green wires.  Call for only the fan?  Connect red and green, etc.  Basic electronics.

The only thing that is an issue now is some smart thermostats need to be powered.  This requires a "common" wire or C-Wire that carries a constant 24v.  If you get a basic non-wifi thermostat, you don't really need to worry about that, so you should be good.

At that point the other major option is if she has a heat pump or not.  If it's not a heat pump, it's much simpler.  If there is, just make sure the thermostat supports all the options she has.

Label the wires or take a picture before removing anything and it should be a piece of cake.