What are reasonable grounds for leaving a job?

Hello all. I'm completely unfamiliar with the welfare system and wondered if anyone could tell me what are reasonable grounds for leaving a job? A friend is under severe pressure due to work and personal circumstances and has been advised to take some time off but cannot afford to do so if it affects his unemployment benefits. Thanks for the help

The disqualification term is leaving a job 'without just cause'. Unreasonable hours, distance/cost fo travel, stress due to work, unreasonable demands by employers are all taken into account in considering this. There is no set reasons laid down as they can be diverse and unique in any given circumsatnce.