MitziK great post, respect to you for an excellent analysis of the challenge facing any entrant to teaching today.
OP, I'm sure you are well-intentioned, but (having worked for 28 years in challenging schools), I think you need to step back a bit and understand your own role in 'the system': we have a massive deficit of teachers (particularly males FWIW), and there has been a proliferation of entry routes to the profession, accompanied by mendacious and misleading media campaigns designed to lure as many would-be teachers as possible. The hard work then falls to the school mentors such as you, in addition to your own (no doubt heavy) workload, you need to spend time and effort developing the trainee.
In most other sectors, it is understood that apprentices are in the process of developing skills and knowledge, rather than being thrown in at the deep end, sink or swim, which is what inevitably happens in teaching.
As PP have said, the decision to quit will have been massive for the trainee, yet you appear to be considering only the impacts on you and your colleagues.
I believe that this exemplifies what is wrong with the current system, in which there are no consistent guidelines for interaction between the mentor and the trainee.
I am sure you are a thoroughly decent person, but many teachers I have worked with get a big power-kick from being allocated a trainee, and are neither professional nor developmental in their interactions, often leading to conflict, disappointment on the part of the trainee and loss of a potential teacher.
One ex-colleague in particular, whose own subject knowledge was severely lacking and who used sarcasm and downright cruelty to the students as her modus operandi, thought she was clever to have 'failed' three trainees on their placements. The reality was that they were not b*tches like her, and their knowledge was better, good enough to know how lacking she was and therefore not prepared to be found wanting by her.
What I mean by 'the machine' is that you are being used by a system which is actually not fit for purpose. I am sure you are nothing like the colleague I have described, but unfortunately I witnessed this type of problem repeatedly, and my sympathy was usually with the trainee.