No equipment available is different from no crew available. It sounds like it was indeed an airplane issue, probably cancelled somewhere earlier due to maintenance.
As far as crew shortages go, this is not the crews fault per se. A couple of years ago the company furloughed a lot of people. Since that time they have also had a huge number of retirements (pilots and f/a's), and they haven't replaced any of them with furloughed employees. Therefore, you have the crews already filling in the gaps for the folks that have left. They also have the crews flying up to 95 hours a month now as opposed to 80-85 that we were flying before, so most of us can't fly overtime even if we wanted to. Pilots can only fly 1000 hours a year and have to pace themselves accordingly. Crews are also limited in the fact that they can't fly more than 30 hours in 7 days so we're constantly having to work around that as well.
And last but not least, no, the airlines do not cancel flights at the drop of a hat just because of a light load. I've flown several flights with one or two people on them over the years.
Sorry she had problems getting home, but I'm glad the delay wasn't more than a couple of hours.