I don't believe there's a setting that affects that directly. I do know the length of each ant varies depending on whether you have the Use OpenGL Drawing setting one way or the other. Also, while you're selecting something the ants will have a slightly different appearance than after the selection is made.
Is it possible one of you has a display monitor with a lower pixel density than the other?
Can you post an image of what you see? Here, for example, is what I see in Ps CS5 while making a rectangular selection, and then when I let up the mouse:
Were you looking at the exact same image, and at the same Magnification, on both machines?
Looking at different Images can fool the eye/brain, into thinking that it seeing something, that it is not.
The area of the Image, where you have your Selection, can make the "ants" seem brighter, or dimmer.
Also, monitors and monitor calibration can yield different results.
Just some things to think about.
PS - I have always wanted to be able to toggle the color of the ants from say red to green to blue, so that they stand out better, depending on the background. Often, the gray can get a bit lost.
Here are a few examples of what I am talking about: http://en.wikipedia.org/wiki/Optical_illusion#Colour_and_brightness_constancies
I cannot find some great ones, involving color preception changed by surrounding colors, but will post that too.
Note: still, his ants might be brighter than yours!