It's exceptionally, exceptionally rare for a student to be chopped unfairly. There's such a phenomenal paper trail, and generally he has to fail a defined training objective; so there'll be a handful of sortie reports from various, progressively more senior, QFIs explaining that he isn't making it. He'll be offerred remedial flying and all the help possible, and if it still isn't working he'll be presented to a withdrawal board.
The withdrawal board will be staffed by QFIs who haven't had any real contact with the poor choppee, and they'll be given copies of the student's recent reports and a specifically-written end of course report. The student himself sees this end of course report and is allowed to comment on anything within it if he thinks its unfair.
He then gets to speak at the withdrawal board, and present a written statement explaining poor performance and putting forward his side of events. He can even list preferences for restreaming or rebranching.
The final decision from the board goes forward to Wing Commander level. No-one pitches up to work to be told "sorry, you've been chopped," and if anyone thinks it's unfair they have several stages at which they can protest. If they appeal, they'll often be reinstated unless there's a robust paper trail proving he's not up to it.
Unsurprisingly, most students aren't keen to tell you they were chopped because they were rubbish, and that's why there're dozens of stories of unfair choppings, witch hunts, personality clashes etc floating around.