Are you telling me personal petty revenge isn't a certified approved bank reason?!

Anti-discrimination laws in the US apply to discrimination based on what's called a "protected class," which essentially boils down to a small set of personal characteristics you're not allowed to base a decision about a person on. For the purposes of housing, this includes race, skin color, and national origin, gender and sexual orientation, whether there are minor children in the family, and disability.

"Being somebody who the loan officer has a deeply personal vendetta against" is not a protected class, and if the original OP did in fact reject a mortgage application on that basis they'd most likely be legally in the clear. Whether their employer would be happy to know about it is another story, but if it was anywhere near a coin-toss decision I doubt they'd ever have to justify themselves.