The United States CBP only collects arrival and departure records from the operators of airlines and passenger ships. As there are no exit checks when leaving the USA, this means that if one leaves the USA via another mode of transportation (i.e., by land), one's departure is not recorded in the CBP's computers. So it makes sense that the US has no record of her departures.
However, your girlfriend's arrivals in Canada should have been recorded by the Canada Border Services Agency. (I will echo Jim McKenzie's skepticism that her passport was never once scanned upon entering Canada; whenever I've crossed the land border, I have always had to hand my passport to the agent for scanning.) So if your girlfriend needs proof that she left the US at the appropriate time, this would be a way to get that information.
Instructions on how to request a Travel History Report can be found at the CBSA's webpage. It is also possible to request a Highway Passage Report, which gives the records of when a particular car crossed into Canada; however, for obvious reasons, this can't be viewed as airtight evidence that your girlfriend actually left the country at that time.
A Travel History Report is a record of a traveller's entries into Canada. The report documents entries made on or after August 1, 2000. This information is collected by the Canada Border Services Agency (CBSA) on travellers entering Canada. Traveller exit information is also available in a limited capacity but applies only to foreign nationals entering the United States from Canada on or after June 30, 2013.
Highway Passage Reports may also be requested; however, they indicate only that a specific licence plate has been recorded, and do not show the people travelling in the vehicle.
Instructions on how to apply can be found on the linked webpage. Essentially, she has a right to this information under the Access to Information & Privacy Acts (the Canadian equivalent of FOIA), and the Canadian government maintains a web portal for ATIP requests like this.