The proper way to use the word would be "the נפטרת was נפטר" in Hebrew I think it would be נפטרה הנפטרת
But you were the OP who corrected nifteres to niftar, by your current rule nifteres shouldn’t have been corrected.
avromie7 is using nifteres as a noun, referring to a person. In English, "the deceased".
His Hebrew sentence could have been written either נפטרה הנפטרת or הנפטרת נפטרה (commutative law of dikduk?) meaning the deceased (nifteres, noun) has passed away (niftara, verb).
His English sentence says that the deceased (nifteres, noun) was (verb) deceased (niftar, adjective).
Now in Hebrew we conjugate adjectives, but in English we don't, and the person is speaking English.
Similarly: Can a woman be yotzei with a man's bracha? Or would she be yotzeis? Or, if it happened yesterday, would you say she was yatza?