Observed this situation play out on a flight last week.

Background: Turbo prop plane, three across. A is to the left of the aisle, B and C to the right.

6B and 6C were occupied. A lady comes on, points at 6B, and says "I think you're in my seat".

The guy there replies "I would like to sit by my friend, would you mind moving?" He points to 7B.

She agrees and sits down one row behind him. Plane takes off.

About 30 minutes into flight, 6B gets up to use the bathroom. Lady moves up to 6B. 6C objects but she ignores him. 6B gets back and looks confused.

Argument ensues. Flight attendant comes and checks boarding passes and tells the guy to move to 7B.

I am a bit torn on this one, so I want your opinions. On one hand, it was her assigned seat, but on the other hand, she willingly gave it up.