Shouldn't the dispatcher have given more precise directions than "follow the river"? If they had said "take a right and follow the river", this wouldn't have been a problem. I suppose they were used to professional pilots knowing what was meant, but they were already working to fit a small plane into a larger system presumably filled with mostly larger jets from commercial airlines that flew in and out of that airpor…
Honestly, I'm surprised by "follow the river" as a flight instruction. That sounds like the directions you'd get from someone that doesn't understand cardinal directions. "Turn right at the gas station, then turn left at the big oak tree" or something. I have no experience with flying, but I assumed it would be a heading / cardinal direction and not "turn right at [landmark]"
https://www.google.com/maps/place/Ronald+Reagan+Washington+N...
Left is upstream and away from DC. Right is towards restricted airspace.