So, I understand how you feel completely.
What could it have been? Well, here's a common list, and I'm not saying that any of them apply to Randy, just that these are the common things:
Behavior. Sometimes a person accidentally, or not, acts in a way to raise a dog's hackles, so to speak. They move in a shifty way, or they wave their hand in a 'be quiet' motion, etc. Or, the person teases them in some way, and the dog doesn't want anything to do with it.
Clothing. Sometimes people wear something that upsets a dog. A hoodie with the hood up, a hat, or anything that a dog finds unusual, or reminds them of someone that they don't like, etc.
Scent. There could be something on the person that the dog finds very offensive, even if you can't notice it.
Substance. This can also come under scent or behavior. If a person takes medication, or drinks alcohol, and the dog has never been around it, or has had a negative experience with a person doing either, they can be upset by it.
And, finally, intention. I want to point out again that I'm not focusing on Randy, just making a list. Dogs can pick up better than we know on people's intentions. So, if someone is a thief, would harm someone, etc., some dogs can pick up on that in various ways.
That's pretty much my list. I lean toward behavior or clothing.
Perhaps, when she first started to bark about something, he told her, physically or with body language, to stop, and she found that threatening?
Is he taller than anyone else that was there?
Were her hackles up?
I don't know either, but think about the list, point by point, and see if anything rings a bell, looking back.
Love All Life, Thank You For Posting! :)