She just states it as fact. I believe her now, because she has been so consistent with her story for such a long time.
That doesn't necessarily mean it actually happened, simply that she believes it did. Also, she may have woken up after receiving sedation but before intubation and the commencement of general anesthesia.
Does she remember being nasally intubated during the procedure? If she says she wasn't, she may not be correct that she woke up during the surgery and not beforehand.
In any case, if she is convinced she woke up, there's probably not much you can do to change her mind, and as long as she doesn't feel anxiety or upset about it to the degree that she might one day refuse a necessary surgery because of it, there's probably no reason to argue the point anyway.
I woke up during a D & C, but since I had eaten that day, I wasn't given general anesthesia, simply conscious sedation. I remember waking up and saying "ouch!" My doctor (not the anesthesiologist) felt really bad about it, but I didn't think it was that big of a deal since they put me right back under very quickly.