Just a thought - on numerous tests and lessons over the years I have heard some phrase along the lines of "and I will be responsible for compliance with the low flying regulations".
Depending upon what your student said, it's possible he was simply waiting to be told to go around? Let's face it, in flying a PFL, their mind should be on the reality of the exercise, not on the artificial aspect that has them put power on once the field is clearly defined.
G