Is that the proper term for it, cuck-inflation? 'Cause I've been watching over the years as the general understanding of the term drifted from 'infidelity in a serious confirmed relationship' gradually toward 'anyone anywhere is involved with someone other than you' with mounting bewilderment.
That’s probably not the proper term for it because I made that up. I intended for it to mean exactly what you described, so I guess it got the job done.
Does it count as cuckoldry if the observer wasn't in a relationship with either of the participants? Trying to figure out whether or not this is a 'poorly-disguised author's fetish' thing, or just a blatant author's fetish thing.
Before the cuck-inflation, wasn’t that just voyeurism?
One who has a monogamous relationship who likes to voyeur the partner having sex with others and gets sexual gratification of it. Here the protagonist has no sexual or romantic relationship with either of her close friends, so the answer is no.
Like Thomasina said, this is voyeurism, which can be someone's fetish as well.