Nope, still don't see it. Women tend to go to the bathroom together far more frequently than men do in my neck of the woods. It's a stereotype, but I don't see how it's a harmful one to perpetuate, unlike your example. Would it bother you if people assumed you wanted to go to the bathroom in a group? If so, is this a common concern?
Are you offended that the author is making fun of certain girls (the particular girls who go to the bathroom together) or that the author is making fun of girls (a group in which a subset go to the bathroom together, and you don't want to be associated with that subset)?
Irate callers are just like those black people who don't tip at restaurants.
Now do you see it?
(Obviously, I made that up. Apologies to Geek Squad and their CEO who I am sure would never say such a vile thing).