- We were speaking quietly not to wake up Mr. Smith.
- We were speaking quietly / not to wake up Mr. Smith.
- We were speaking quietly in order not to wake up Mr. Smith.
- We were speaking quietly / in order not to wake up Mr. Smith.
The problem is not in the choice of words. It is rather in the construction of both of these sentences, which are lacking a simple comma, as using the adverb not at the beginning of the sentence modifier seems to form a conditional clause.
You may have noticed that the adverb clause of condition gives the circumstances under which the action in the main clause will take place.
Both sentences are phrased with the main clause and additional adverb clauses, which would have been better constructed this way:
- We were speaking quietly, not to wake up Mr. Smith.
- We were speaking quietly, in order not to wake up Mr. Smith.
In order is really a little superfluous, but that's just a question of style preference. However the placement of not at the end of the main clause, especially in the absence of a comma for separating the clauses, creates some confusion because in order to make sense it should be paired with the infinitive phrase in your adverb clause (i.e. not to wake).
Dropping it right in the middle of the sentence between the main verb and the infinitive phrase within the adverb clause, without punctuation, might make the audience wonder which clause and which verb it was intended to modify. So it's understandable that your friend insisted on using another phrase, in order, in an attempt to form some sort of logical separation between the clauses of the sentence.
Ironically, a simple comma should have been sufficient.
Yet the sentence could also be improved with a different placement and usage of the adverb not modifying wake, without the comma:
- We were speaking quietly to not wake up Mr. Smith.
- We were speaking quietly in order to not wake up Mr. Smith.
To wake, or to not wake, that is the question. It is not a split infinitive, when to is being used as a preposition. And the comma isn't necessary to separate the prepositional phrase following the main clause.
Word Origin and History for to
preposition: Old English to "in the direction of, for the purpose of, furthermore," from West Germanic *to (cf. Old Saxon and Old Frisian to, Dutch too, Old High German zuo, German zu "to"), from PIE pronomial base *do- "to, toward, upward" (cf. Latin donec "as long as," Old Church Slavonic do "as far as, to," Greek suffix -de "to, toward," Old Irish do, Lithuanian da-).
The nearly universal use of to with infinitives ( to sleep, to dream, etc.) arose in Middle English out of the Old English dative use of to, and it helped drive out the Old English inflectional endings (though in this use to itself is a mere sign, without meaning).