The conclusion was reached most likely through the theory of discounting and augmenting.
In the theory of discounting, we tend to cancel out the potential offender leaving us to the conclusion of blaming the other. Since the she has never seen her friend act that way, she already discounts this behavior to be reasonable. Thus, she discerns that it must be the sales clerk's fault that made her friend act that way.
In the theory of augmenting, judgement is given to the possible cause that lead toward that unfortunate event of misunderstanding. The cause refers to the sales clerk provoking her friend to act that way.
These two theories are principles that focus on establishing the cause .