"She sees her boss as a father."
Translation:Sie sieht ihren Chef als Vater.
There is no need for the indefinite article in this sentence in German. The sentence sounds perfect without, whereas, if you omit it in English, the sentence sounds odd. So there is a good reason to replace the literal translation ("....als einen Vater") with a more common variant.
der Chef is singular masculine, and the ending possessive pronoun is matched to the word it refers to, in gender, number and also in the case, which is accusative here (like most direct objects), while "ihr" would be nominative case, singular masculine. A correct sentence (not a direct translation, though), would be "Ihr Chef ist wie ein Vater für sie" (nominative case here, as the boss is the subject).
then why was my answer marked wrong, "Sie sieht ihren Chef wie einen Vater"?
another question: Duolingo marks me wrong often when i omit an indefinite article in the German translation, even though there is no indefinite article in the English version. Does the German language insert & omit articles at random? i've been unable to find any rules or patterns to help me navigate this problem.