Thanks, all, for your contributions here.
The woman mentioned in the opening post dropped our email conversation after I basically told her that, as I see it, her guy isn't treating her well -- and to such an extent that I encouraged her to consider moving on.
She never explicitly said that she wanted to have another man in her life, but did make it clear that that would be unacceptable to her guy, who is clearly looking for another woman and wanting his gal's support in that. I suppose it's possible that she doesn't even want another man in her life, and that the subject only emerged into conversation because he is actively seeking another woman.
His attitude and expectation seems pretty self-centered to me, and may even represent
a throw back sexist thing where "a man should have more freedoms than a woman" (as the story went). I'll probably never know the details of their situation, however.
[If you're reading, gal in question, I truly wish you the best.]