Earlier quoted context omitted.
There was a user action to install it, but it's pretty sneaky. MS changed the behavior of the upgrade question dialog that if you close it with x it will start the download and then install when ready. Your girlfriend must have clicked the x some time prior to the sudden upgrade. Even my tech-savvy coworker who didn't want to upgrade got caught out by this. Anyway, you can still roll back, and then install never10.
Edit: After some thinking: Yes, i agree this is a dark pattern and MS needs to be held accountable to it accordingly. My only disagreement with the current direction of discourse is that people are not saying "MS manipulated me into an update", which is the truth, but "Windows updated on its own and there was nothing anyone could do", which isn't. -- They didn't change it, it was always that way. People only realized…
As parent (+1'ed) notes, the message itself is quite clear in its intent. I would not for a second think that closing the window would infer the "cancel path" as the result.
Yes, they are being too forceful, and Microsoft ought to have given users a clear "I do not want to upgrade, ever" option, yet at the same time Windows 10 represents a significant increase in security, and in these days of massive botnets and what not that is a good thing on a big scale. In a roundabout way, I'm picturing this as forced immunisation for the greater good.