It has to be 'a secretary' here, because the lack of an article in the Spanish version means the same as 'a secretary' in English. If it was supposed to be 'the secretary, then it would have been 'la secretaria' in the Spanish version.
As others have noted here, you don't use an indefinite article when talking about someone's profession. However, if a definite article is called for in Spanish when talking about someone's profession (e.g. Él es el ingeniero), then you don't drop the definite article.
I just had an "ahah" moment i thought I'd. I too was confused by the presence of 'fui' thinking it is from 'ir'. I had forgotten that it is not only the preterite past of ir, but also ser.
In other words, 'I was' and 'I went' are the same when using this tense.
The indefinite article is dropped when describing a profession or occupation unless it is modified by an adjective.