"elle est allée" is passé composé, as well as "ils sont allés" or "elles sont allées".
with auxiliary "être", the past participle agrees with the subject.
with auxiliary "avoir" the past participle is invariable except if the direct object is placed before the verb:
il a rangé ses chaussettes
ce sont les chaussettes qu'il a rangées (qu' is a relative pronoun, replacing chaussettes, fem plural)
Does this need to be reported? As indeed it seems (to a novice - me) that this should be passé composé as apposed to imperfect.
Is this correct: Elle allait au café = She was going to the cafe (...when something happened). Elle est allée au café = She went to the cafe.
However, I am not sure if this is an oversight by Duo or an exception to the rule?