fat lady vs airline seat

OK, I know this is rude, but it is a serious problem.

There was a lady that was easily 400 lbs on the airplane with us. I watched her board before everyone else.....which spared anyone who might have been sitting on an aisle seat as her passing would have deprived them of whatever was near the edge.

I watch her get in her seat. She doesn't fit, so she moves the arm rest up to fit and cannot put it back down. I watch as people board the plane. She has to use the restroom, and she has to back into the bathroom as there will be no room for turning once inside. She doesn't lock the door, and some unexpected gentleman walks in on her. He is, most likely, at least partially blind, now.

She returns to her seat to find that two other young girls have now occupied their seats next to her. She attempts to squeeze back into her seat, but cannot get in without much effort, then there is not room for her as she occupies almost the entire seat next to her.

Luckily for the two girls next to her, the plane was not full and they were moved. Why didn't this huge person have to buy two tickets? What would the airline have done if the plane were full and she wouldn't fit in her seat?

Another thing, the airlines are now nazis about how much your luggage weighs. If it is even one pound over, you must remove it or pay extra. Me and all my luggage, plus my flight companion AND his luggage, didn't weigh at much as her alone. If you have to pay extra for luggage, she should have to pay extra for weighing down the plane, using two seats, and causing the plane to use extra fuel. Oh the injustices.....