I would consider the story plausible. I have heard stories of elderly ladies being fined for DUI after being given the wrong drug by a malpracticing doctor and causing an accident. The courts are not a forgiving place.
This I do find plausible. The difference is that the DUI did happen, whereas in the cancer medication case, the pill was not actually taken. There is a legal defense in situations like that called automatism that, for various reasons, is rarely used, even in cases like this. Prejudice against elderly drivers may factor as well. You can read more about automatism here: https://en.wikipedia.org/wiki/Automatism_(law)