Mirror mode with Dell 2410 problem

Mirror mode will only allow you to use the max resolution of the MBP's display, as it cannot display a higher resolution than that. This is completely normal behaviour, the MBP's display cannot display more pixels than it physically has.

If you want full resolution of your external, you'll need to be using either clamshell mode or an extended desktop.

 

Nope, not the graphics card. It is equal or lower resolution than the laptop.

 

I do believe that Windows allows the adjustment of the secondary monitor's graphics even in mirror mode. it has nothing to do with the graphics card but how the OS was written to handle this function, and that is how Apple decided to handle it. We are not sure what laptop your gf has anyway and it is possible that her main monitor has the same resolution depth of the 55" display (as with Dell I know they offer a 1920x1080 display option). If Apple had that then you would see the 55" be mirrored as well.

 

I can confirm that my windows 7 does not allow me to do this.