How long has he been in that job? If it's at least 2 years then he has some protection from being unfairly dismissed if he raises the stress as an issue with his employer. Take steps to reduce the stress: Employers have a duty of care to protect employees from stress and do an individual risk assessment and action plan to address this. More here: https://www.acas.org.uk/managing-work-related-stress/understanding-the-law Take some time off work to recover and reevaluate: He should also check his company's sick leave policy and talk to a doctor in case he eventually needs a sick note to get signed off work. It might help him to take a brief break as sick leave. If you don't have an emergency fund, I wouldn't recommend him just resigning with no clear plan. If he resigns, he probably won't be entitled to benefits while he looks for work, so that really should be a last resort after exhausting all options. If he's confident he can find something else fairly soon, then he should get applying!
If he can, he could go sick. But be careful as it affects future prospects. He needs to proactively search like crazy for a job he prefers. I understand how much you want to support him, but walking out isn't a decision he should do lightly. Maybe he could talk to a doctor. Maybe it's not just work stressing him out? Dig deeper. But regardless he needs to get another job before leaving. Good luck. I've been here. It makes it harder x