That's a cool idea, Piper!
My very first lesson horse would carry his tail off center if the rider was off center. I remember my first riding instructor telling me to sit more to the right, since Max's tail was sticking out to the right. Not putting enough weight on the left got a leftward sticking tail.
Most showing disciplines will penalize a swishing tail, since it can indicate resistance to the rider's cues. Usually, though, the tail swishing isn't the only resistance the horse shows, it's only part of a picture that includes pinned ears, raised head carriage, gapping of the mouth, full body stiffness, etc. Since it sounds like your friend's horse was really well behaved, I'd chalk it up to flies.