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".