If she used the hotspot to connect the brothers computer to the internet for the first time in a while, I am betting it was OS background updates. 2GB of updates isn't uncommon for OS X or Windows over the period of a few months to a year.

And Doug, price wise, unfortunately that seems about right for the device and 2G of data with someone like AT&T. In the future she may want to investigate using such "personal hotspot" options on her phone if she has one capable of offering the feature. Would save on the cost of a physical device at least. Be warned though using these options on a phone tend to drain them quickly compared to the dedicated devices.