Of course, you only want what's best for him. That's completely understandable.
I've just double checked and that particular firm is definitely not small. Currently what I am hearing from people who work there is that they are miserable and stressed doing 50-60 hour weeks. I work in a smaller firm with a different culture. Other than in exceptional circumstances our trainees work their contracted hours and there are certainly no expectations to be out socialising frequently.
However, repeated exam fails would cause concern anywhere because it suggests the person may be on the wrong path and will struggle to qualify or reach the required standard. Training contracts are also so expensive they are loss-making in the first years for the employer, so fails increase that cost. After the first fails, we would have had a discussion to understand what the issue was, if support was required or if this wasn't the right path. We do have a termination policy for repeated fails or bad fails. I would expect any firm offering training contracts to have such a policy.
If he still wants to pursue accountancy and has learnt from and resolved the mistakes he made with the previous exams (e.g. it sounds like he under-estimated then and didn't do enough serious preparation), then I'd suggest looking at firms in perhaps the top 60, 50 and 40 and assessing their culture to find a better fit. I would expect most firms in the Top 20 to present similar (or worse) stressors to where he was, so I wouldn't be looking there.
It would be sensible to get a pass for the latest failed exam secured under his own steam to prove himself. He would probably be looking at semi-senior roles and it might also be worth connecting with an agency specialising in accountancy roles.
If he was working in audit and finding the intense client facing nature of that role tough, it might also be worth considering more "back office" roles with less client contact, e.g. accounts preparation, outsourced services, or part of the finance team in a practice (i.e. doing the accounts for the accountancy practice itself rather than for their clients).
If he doesn't want to pursue accountancy anymore then no shame in that. It doesn't mean he won't be successful in another field - I've certainly seen people who struggled to pass their exams, realised accountancy wasn't for them/what they imagined and went on to be successful doing something different.
Lastly, as an ICAEW student he is eligible to access support/services from CABA so that may also be worth exploring: https://www.caba.org.uk/