"I didn't realize it was him." [duplicate]

Overheard on an elevator today,

I didn't realize it was him.

Corrected by the know-it-all,

He. "I didn't realize it was he."

The know-it-all then went on a rant about how everybody is a dolt for not knowing that.

I swear I have never heard this before in my life. Is the know-it-all correct?

If so, why is "I didn't realize it was he," grammatically correct? What rule makes "him" incorrect here?

The 'rule' is that the verb to be, in any of its forms, is a copula that takes a predicate rather than an object, hence "It is I" and other phrases beloved of English teachers in junior schools. There are various theories about the reasons for this, but the truth is that it has become a shibboleth to distinguish those who had 'a proper education' from the hoi polloi who just learned English as they went along. [Another such is to point out that hoi is the definite article in Greek, so 'the hoi polloi' is a tautology; please don't comment to that effect unless you wish to be mocked.]

Nobody pretends that this rule is used in everyday speech; Barham's 'Jackdaw of Rheims' has a distinguished audience of prelates and clerics observe a jackdaw

His eye so dim,

So wasted each limb,

That, heedless of grammar, they all cried, "THAT'S HIM!--

showing that, though you are supposed to know that the 'correct' phrase is "that is he", you are not expected to use it in moments of stress.

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."

No. The know-it-all was misinformed. Not terribly surprising, I spose.
Peevers are just poor people who've been bitten by a zombie rule.
He might as well go on a rant about how the singular of you is thou,
and anybody who doesn't realize this is a dolt.

The English nominative personal pronouns (I, he, she, we, they) are now only used as subjects. The objective personal pronouns (me, you, him, her, it, us, them) are used elsewhere; in particular, they're used for anything following the verb, including predicates.

Just like the use of French moi, for pretty much the same reason: loss of the case system.
C'est moi or C'est je? Luckily, the French don't seem to have our zombie rules,
so they don't have to say that "technically, it's c'est je".