"She is my captain, and my friend."
Translation:HoDwI' jupwI' je ghaH.
I may have been confused by the comma here, and wrote HodwI' ghaH 'ej jupwI' ghaH because I interpreted it as "she is my captain; and she is my friend" rather than "she is my [captain and my friend]". Is the Klingon version correct as per my interpretation, or is it wrong under all circumstances?
Your sentence is not grammatical Klingon because jupwI' is a noun. A Klingon sentence requires a verb to be complete. By using 'ej you are required to provide another sentence but you only provided a noun. You could say HodwI' ghaH 'ej jupwI' ghaH. You need that last ghaH to act as a verb so that the last piece is a grammatical sentence on its own.
But that translates to "She is my captain and she is my friend" which contains an extra verb in the last piece which is not in the English sentence so I wouldn't recommend that translation because the answer sentence really matches the Klingon better. Although both are correct translations.