When we use but as except, we use object pronouns after but (you, her, me, us, etc.) even in subject position: No one but her had well prepared for the dance.
In formal situations, we can use subject pronouns after but:
Everyone but he knew how the events were going to unfold and the issues were starting to emerge.
Now, using ‘except’ for ‘but’ in resolving ‘she’ or ‘her’ following ‘no one but’ pattern clears this anomaly well:
All the prisoners except Timothy started to complain about the poor quality of food. (The statement does not include Timothy).
No one but I saw the movie.
No one but me saw the movie.
Primarily, both versions are acceptable and most grammarians are of the view that but is both a preposition and a conjunction. However, remember that conjunctive but followed by a nominative pronoun seems rather literary than the preposition. It may be only stylistic choice where the use of nominative ‘he’ version might be considered to be rather formal in style. So, it is up to you which way you use it.
Everybody but him has failed the exam.
Everyone but he avoided speaking to the media about the current economic situation in the country.
All the candidates but she are the best candidate for the job.
All the candidates but her are the best candidate for the job. (except for her, excepting her).
All but one
Of 20 aspirant dancers, all but one worked very hard to achieve professional status.
(but means except meaning 19 aspirant dancers worked very hard…).
The adverbial phrase all but (without hyphenation) also means nearly, almost, or on the verge of: His paintings are all but vivid in their detail and accuracy.
All but indicates that the following action is almost complete but not absolutely. If we say:
He all but has finished the dinner. (= means there is a small amount of food left to eat).
All but can also mean all except: I have written all but the conclusion of the essay.
She waved me back once and all but disappeared into the crowd.
After several hours of deliberations, labour union and the owners all but reached a compromise.