Dr. Montessori was pretty clear about what kinds of behaviour should be stopped and what kind of behaviour should be left alone. For example, in The Montessori Method, she said, "We must, therefore, check in the child whatever offends or annoys others, or whatever tends toward rough or ill-bred acts. But all the rest, -- every manifestation having a useful scope, -- whatever it be, and under whatever form it expresses itself, must not only be permitted, but must be observed by the teacher." (p. 93). I have interpreted this to mean at all times including during false fatigue.
So, if the imaginary play is not annoying or offending other children and the materials are not being misused, I would stand back and observe what is actually taking place. Is the child exploring the materials and making discoveries within that imaginary play? Does the child abandon imaginary play after a few minutes and become deeply focussed on the work? Is this play the child's way of partaking in false fatigue – is this her way of preparing for the next stage of work? Observation will answer any of those questions and will dictate what, if any, action is needed by the teacher.
Hope this helps.