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I was hoping for some help to resolve a disagreement over the wording in a specific sentence.

"Most importantly, she is one of the select few whom I acknowledge as friends."

Yes, the tone is intentional. I prefer "as friends" while she wants it to be "as a friend." As far as I can determine, "as friends" is the grammatically correct way to phrase it since "the select few" is clearly plural, but she claims that it just doesn't sound right.
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I would say as a friend as 'She is one' is the subject of the sentence. I understand your logic but disagree with it. I'm not a teacher so check for answers from some of the others.
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Falconer"Most importantly, she is one of the select few whom I acknowledge as friends."
I think both "a friend" and "as friends" or even " as my friends" are very possible. syntactically, " ...the selected few.." suggests "among the people I feel most comforatble with and trust".

So...." She is one of the select few whom I acknowledge as friends", signifies common friends who hang out together.
Then..

" She is someone whom I would like to consider as a [special] friend some day". "A friend" is a more defined figure than "as friends". That said, based on the original context, I'd say "plural" sounds more logical. But that's only my perception.
As I see it, "one of the select few" is the DO of the predicate clause ("I" being the subject). Come to think of it, that seems to suggest that it should be singular. It does seem like an argument could be made either way. Interestingly, I ran into much the same problem when I tried the phrase in Swedish.