Not that I want to defend her, but in a modern car where the gauges are always illuminated and it has bright DRLs up front, and driving on what I presume was a well-lit motorway, I think it would be quite easy to miss that your headlights weren't on. Even if you're in traffic, you might see the reflections of your own DRLs on cars in front and assume the lights are on. Obviously it's not an excuse, as any competent driver should be paying attention to that, but modern cars don't really seem to be making things easier.
I'll even admit having done it myself. Years ago, not long after I first started driving, I dropped a mate home late one evening. Pulled up outside his house and flicked to sidelights while I was parked up. He got out and I drove off - the few miles home through town was on completely street lit roads and given that I could see fine and my dash was lit up, it didn't even occur to me that my headlights weren't on until I got within a few hundred yards of my house. Granted this was a Mk1 Focus s the lights weren't anything like modern cars, but that only exacerbated the problem; they weren't so bright that I'd easily miss them under street lights.