Technically, the know-it-all is correct. When a pronoun falls after some form of the linking verb to be (am, are, is, was, were, being, been), then it should be in the same case as the subjective pronoun. As Claire Kehrwald Cook states in her excellent book, Line by Line (p. 182):
"Since this verb [the to be verb] functions only as an equal sign, a pronoun that follows should logically be in the same case as its equivalent on the other side of the equation. Ordinarily, then, the pronoun to the right of be belongs in the subjective case. The following sentences are grammatically correct: It will be you and I who suffer the consequences. It is they who are responsible.
However, Ms. Cook goes on to say this about everyday usage:
In informal contexts, though, the grammatical rule is often set aside. Most usage guides, in fact, consider It's me and That's him acceptable and even preferable in general use, where the strictly correct alternatives would seem stilted.
After the infinitive to be, the objective case is usually both idiomatic and technically accurate. They expected me to be the winner and They expected the winner to be me are both good grammar, since the subject of an infinitive must be in the objective case and a subject and complement should match. If the infinitive does not have a subject, however, the complement is in the subjective case, matching the subject of the main verb: She wished to be I. That sentence, of course suggests greater formality than anyone recommends for ordinary purposes. "I'm so lucky to be me" goes the song from On the Town, and no one faults the lyricist.
The rules governing I and me also apply to the other personal pronouns with distinctive subjective and objective cases: he, him; she, her; we, us; and they, them.
So, I would say, that the know-it-all was correct. As an example, I personally do answer, "This is she" when someone asks for me when I answer the phone. Honoring that linking verb. However, in informal contexts, I wouldn't think anything of someone saying, "It was her."