She's next door.She's in her office,sir.
she在里面是主语,应该用主格,不能用her.如果是他,也不能改成His is next door.His in his office,sir.应该是：He is next door.He is in his office,sir.
主格/宾格/形容词性物主代词/名词性物主代词
I/me/my/mine
you/you/your/yours
he/him/his/his
she/her/her/hers
it/it/its/its
they/them/theri/theris