I think using shortening instead of butter makes them softer, too.
But if you two are using the exact same recipe, it probably has to do with how long you leave them in. Also, are you using a dark pan? Aluminum pans will make your cookies brown more slowly, which would help.
Another factor might be your oven temperature. I had a problem with all my pie crust edges burning, and bread crust getting too brown, and it turns out that my oven runs 25 degrees hotter than the knob says. If you have an oven thermometer, you can see if your oven temperature is accurate.