Choosing a grammar book or rule to follow depends on the situation and how strictly you adhere to traditional grammar rules. Some modern grammar books may allow more flexibility in interpretation, considering constructions like "It is said that..." to be a type of passivization for practical purposes, even though it does not follow the classical model of passive transformation. If the goal is to stay aligned with traditional grammar rules, then your understanding that intransitive verbs cannot be passivized would be correct.
The sentence "He is an honest man" uses the verb "is," which is a linking or copular verb connecting the subject "he" with the subject complement "an honest man." Traditional passive construction typically involves sentences where transitive verbs are used, that is, verbs that require a direct object to complete their meaning.
The phrase "It is said that he is an honest man" is not a direct passive equivalent of "He is an honest man." Instead, it is a way to report a statement and does not transform the original sentence into a passive voice in the traditional sense. Rather, it introduces a reporting clause, "It is said that..." to convey the same information indirectly.
Since your friend shared some references, here are some that support the traditional view:
- A Comprehensive Grammar of the English Language by Randolph Quirk,
Sidney Greenbaum, Geoffrey Leech, and Jan Svartvik.
- The Cambridge Grammar of the English Language by Rodney Huddleston and Geoffrey K. Pullum.
- Practical English Usage by Michael Swan.
These references should support the traditional understanding of passive constructions and clarify why the sentence "He is an honest man" would not be considered passivizable by their standards. They would likely categorize "It is said that he is an honest man" as a reporting structure rather than a passive voice transformation.