Translation:I have known him for a long time.
Sentence final 了 does not indicate completed action. That is verb-了. Rather, sentence 了 indicates that the comment is currently relevant. In this case, the speaker is saying that, at the moment of utterance, she has known him a long time. To clarify the function of sentence 了, compare the following two sentences: (1) 我学汉语学了三年。and (2) 我学汉语学了三年了。Sentence (1) indicates that the speaker studied three years of Standard Chinese in the past but is not longer learning the language. Sentence (2) states the same period of study but indicates that the speaker is still currently learning the language. This change in meaning comes from sentence-final 了.
It is a good idea to use a dictionary or google translate while doing these exercises because a single Chinese character can be a verb, a noun, an adjective, and an adverb.
As a noun, 久 means "long time." As an adverb, it means "long."
One of the early phrases we learned in this course was 好久不见 or "long time no see."