it could be any number of factors.
first of all, what type and what speed isp can affect network speed.
some people have multiple apps running in the background from weather to news to alerts which will stack with the data usage from what is done in a browser. youtube, netflix and internet radio apps can use an enormous amount of data as well. downloading at the same time....well you get the point.
the first thing to do is to ask what she was doing on your network at the time and what apps she has running.
its rather easy to download a gig of data in a day or over several days from simple things such as youtube, radio and downloading. this would definitely be an impact on slower dsl