I'd personally choose the first one,
she being
as it emphasizes more on the her. her being would be better of in a context, where possession is required.
This looks like the rules of Latin being applied to English.
In Latin, a subject complement takes the nominative case. In Latin you must say "It I is". In English the objective case is usually correct: "It is me". However attempting to simulate Latin has lead "It is I" becoming an acceptable alternative.
It looks like Norman Lewis is from the prescriptive English grammar = Latin camp. This results in sentences that sound stilted, or in some cases just incorrect grammar. A simple rule is "if the pronoun is the subject use the subject pronoun":
- Your husband doesn't believe that you are older than I
Not incorrect (due to the influence of Latin), but "me" would be preferable to most speakers. ("I" is not the subject so "me" is better)
- Maybe we are not as rich as they,...
Incorrect application of Latin Grammar. Use them
*I can't remember who it was.
Correct. You are rarely wrong to use who in place of whom. Whom is an object pronoun, who is nowadays being used in as both subject and object.
- No one but her ever made a perfect score in the test.
Correct. "Her" is not the subject, so don't use "she".
- We have just interviewed an applicant who...
Correct, as before
Was it she you were talking about?
- Incorrect. Incorrect application of Latin grammar. "She" is not in subject position, use "her".
Best Answer
Mainly on the basis of "grammar rules" more suited to Latin, there are still plenty of pedants willing to tell you accusative pronouns (me/him/her/them) are incorrect in such constructions, and that you should use a nominative pronoun (I/he/she/they).
I suppose the "rationale" for that position is there's a "deleted" verb following the pronoun (more soft-spoken than she is, in OP's example). But you've only to look at the chart in this excellent ELU answer to see that native speakers in general have increasingly abandoned that position, particularly in recent decades. So if you want to sound like a modern native speaker, use the accusative pronoun her here.
TL;DR: I wouldn't go so far as to say she is a "hypercorrection", but it's old-fashioned and stuffy.