"He is a very nice person."
Translation:C'est une personne très sympathique.
You would get the same or opposite phenomenon with "une victime", or "un témoin" or for instance:
She is the only witness = Elle est le seul témoin (or "c'est le seul témoin", which could be interpreted as a "he")
He is the first victim = Il est la première victime (or "c'est la seule victime", which could be interpreted as a "she").
The other way around. Read the comments on c'est vs il/elle est in the Tips and Notes here:
The "une personne" is silly overkill when everyone knows a person is male or female without changing a word's ending from 'masculin' à 'feminin' and we certainly don't need to change from "Il" or "Elle" to "C'est une personne" to re-explain that it is a human being, a person, a female or male.
Why is the gender not important? We are making an assumption that 'he' is acting as a demonstrative pronoun in the Latin grammatical sense, pointing at something mentioned earlier that is obviously a person of masculine gender. In this case the gender would be clear. If the English statement is out-of-the-blue (and the subject isn't present to boot) this assumption is broken. The French given can't be out-of-the-blue because the grammar is Latin-based and a demonstrative pronoun must always refer back to something, no? Or has French changed too? Therefore you'd have to start by giving what is referred to, correct the common English to posh English that obeys Latin grammar such as (considering man/boy makes person redundant) "A man/boy, he is very nice." -> "Un homme/garçon, il est très sympa(thique)."