I was thinking about when a descriptor other than the name or pronoun works for me, and I think it might be a POV issue (of course :). When we're in Joan's POV, how would she think of the other person? I don't mean the actual words necessarily, but the relationship of "me" to "her." That is, consciousness starts with our distinguishing ourselves from everyone else, so the first descriptor would be "other," that is, "not me." So that's why, I think, you frequently see in a passage that's in one character's POV a reference to "the other man" or "the other woman," meaning the non-POV character.
I suspect the reader will have no trouble distinguishing with a term like this, and will have no objection-- that is, if we're seated solidly and deeply in Joan's POV, the reader will go along no protest with a term that Joan would use to describe (consciously or no) the other person.
That's why, with Dave's example, both clg and I thought of describing Podgorny as "her opponent," because that's what she is right then in Sally's POV-- the opponent. If we were so deep in Sally's POV that we were in her voice as well as viewpoint, that term might be more, um, colorful. But the term would describe the relationship at that moment. If Sally and P would both survive the fight and shake hands and go out for a drink together (okay, five drinks) and share sad tales of fights that didn't work out so well, in the scene where they drunkenly stumble back to the barracks, warbling Sting songs in two-part harmony, it might be "Sally and her new friend," or "Sally and her one-time enemy," or "Sally and her drinking buddy." That is, the descriptor would change because the relationship has changed.
(This reminds me of my sons, who were both home this weekend so they're on my mind. When my younger boy-- and see, there I am, using a descriptor instead of his name-- was born, the older one wasn't yet two, and of course he regarded A as an intruder. Once he started talking, he referred to the baby only as "that other little guy." It wasn't until they actually started having a relationship that he started referring to A by name, and then "my little brother." And I noticed this weekend, now they're both theoretically grown, he refers to A just as "my brother." I pointed this out to A-- "He doesn't call you his little brother anymore," and A said gruffly, "He can always call me that. It's okay with me." :)
Anyway, in deep POV, it seems to me that the descriptor should always be something that reflects how the POV character would think of this Other. (Again, this is not necessarily the exact words, as long as we're not "in voice"-- that is, writing the narrative in the character's own voice.") So if Sally initially doesn't know her opponent's first name, the narration shouldn't refer to her as "Olga." But after their drinking time begins, and Podgorny says, "Oh, hell, call me Olga," the narration could start referring to her as "Olga."
In omniscient POV, probably any descriptor that the reader would associate with that character would work, but not more than a couple for each. The goal is to limit confusion, not spread it, and too many descriptors for each character could get confusing. ("Which one is the redhead again?") I'd also suspect that pairing descriptors could get annoyingly clunky ("the redheaded detective") though Homer used them (cf. Homeric epithet).
What about in multiple POV? Well, in multiple, you're always (or should be always) in someone's POV. You just shift within a scene to another POV as needed. So whatever character's POV you're in at that moment should dictate what descriptor is used, and yeah, that can mean "her opponent" or "the other woman" refer to different people at different times in the scene. I think as long as you are deep in a character's POV -before- that point, the reader will follow no problem.
What do you all think?