I had this happen to me. I forced down the arm rest between me and a fatso, and that totally PO’ed the guy. If anyone had to stand, it was not going to be me.
My husband insisted he'd purchased one whole seat not one-third and needed to be seated in first class when attendant stated "flight full."
She said first class was also full and husband needed to find someone who might exchange their seat for his(middle seat.)
Husband kept insisting something be done and asked for the lead attendant. The woman with whom he was interacting said that she was in charge.
We finally found out that young obese woman had a mom and elder sister in back (both rather slim in comparison. Older sister finally exchanged her seat with mom for the one next to us.
I might mention the race of participants: obese woman, her sister and mom were black as was the rude lead flight attendant who pretended that someone taking another person's seat because they are so obese, shouldn't be a problem for "most people."
We had the distinct feeling that if attendant were the same race as we were (white) she might have been more understanding to us.