It sounds like she was incredibly embarrassed
I don't think she sounds embarrassed - she sounds entitled.
She knew she wouldn't fit into one seat, but did not book two
She was given a courteous offer of another seat but insisted on her "booked seat"
She had booked a seat which had less space than some other seats anyway - she could have booked an aisle seat.
She insisted that HER needs take precedence over other people's and wasn't prepared to compromise, even for a few minutes while OP ate a meal.
AND - she made a huge fuss and public fuss about being "fat-shamed"
These are not the actions an embarrassed person. They are the actions of someone wanting a confrontation. I wonder if she's currently suing the airline for not accommodating her, and putting her next to someone who was so "rude"?
There should be a protocol in place for people who are very large, but haven't booked themselves enough space. If she had been drunk or drugged, they would not have let her onto the plane. Had she been in labour, they would not have let her onto the plane. She hadn't booked sufficient space, they should not have let her onto the plane.
It may not be her fault she is fat (but I'll bet it is!), but it isn't the fault of the OP, or any other passenger either. Even if it isn't her fault, it's her problem and she shouldn't be forcing it onto other people.