John was seated on her left.

John was seated on her left.

be seated=sit
eg:
seat,使就座：He was seated.=He seated himself.=He sat down.

John was seated by her left. 〈不确定〉
John was seated over her left. 〈不确定〉
John was seated next to her left.< 不对，left去掉即可>
John was seated on her left.<个人觉得应该这个对,表示在左边>
John was seated to her left. <二楼的说法有一定道理，表示靠左的意思〉参考资料：高考必备

首先这不是被动，只是seat的用法 常用be seated by是正确的

seat v
[Tn esp passive 尤用於被动语态] (fml 文)
make (sb/oneself) sit 使（某人[自己]）就坐,坐下
Seat the boy next to his brother. 让那个孩子坐在他哥哥旁边.
a statue of a woman seated on a horse 一个女子骑着马的塑像
Please be seated, ladies and gentlemen. 女士们、 先生们, 请就座.
She seated herself on the sofa. 她坐在长沙发上.
left n
In Britain cars are driven on the left. 在英国汽车靠左侧行驶. She was sitting immediately to my left. 她挨着我坐在我的左边.
都不对 应该是这样
John was seated to her left.
约翰坐在她的左边