In contacting UA you should mention the part about her moving from one E+ seat to another E+ seat, just to avoid UA thinking this was an E- to E+ move. Also, I'd suggest mentioning that other passengers made the same move without being challenged by the FA. Your fundamental case is that there was nothing wrong with her moving seats and it was very inappropriate for the FAs to treat her badly, regardless of whether other passengers did it. But adding the part about other passengers doing it makes your case a little stronger and the FA reaction even more odd.

Good luck!