The phrase is 'was [a] witness to', except in a metaphorical use of 'became' - when, say, an investigator identifies witnesses (people who 'were witnesses') and then realizes that some other person with special abilities (like a blind person - previously discounted as a witness - with acute hearing) should be used as a witness. or when a defence witness becomes a witness for the prosecution. In these cases, the becoming is a change of status rather than a change of essence.
"Witness" in your sentence is a noun (to answer your question ), and a subject complement.
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