What's the word or phrase to describe this behaviour?

Solution 1:

In psychology, it is called hindsight bias. (also known as knew-it-all-along effect or creeping determinism).

The hindsight bias is defined as a tendency to change a recollection from an original thought to something different because of newly provided information. wikipedia

Solution 2:

This is perhaps not as specific as you'd like, but what the woman essentially did was

backtrack

1.1 Reverse one’s previous position or opinion

or

back-pedal

Reverse one’s previous action or opinion

You might phrase it like: "The woman blatantly back-pedalled."

In an open and unashamed manner:

(Oxford)

PS - Back-pedalling is not a perfect fit here; as the woman is hoping no one notices and gives her credit for Z-line. This isn't implied every time someone back-pedals