The fact that it is alleged the FA called the complainant a 'c-nt' is probably the part that makes me really doubt the story. A downgrade and FA rudeness all certainly sound like realistic scenarios but that word gets me to raise my eyebrow. I've seen a fair few UA FAs act in a less than pleasant way toward passengers and others but never seen them resort to extremely crude insults when they go on their officious power trips.
Even as far as insults go it seems like an odd one given how it's considered so very off-limits in an American context. I know it's now a bit more commonly used due to recent hipster Anglophilia, but that is more likely to be the complainant's social set..