What an employee does outside working hours is of no concern to an employer (or shouldn't be) provided it does not adversly affect his work, this goes for criminal convictions too, so for instance say a shop worker was caught drink driving if his employer was to fire him this would be an 'unfair dismissal', however if a lorry driver was caught drink driving and his employer fired him this would be a fair dismissal. Additionally to your case some protection would be provided to the individual suffering from a mental illness through the Disability Discrimination Act, if the individual was suffering from a long term and clinically well recognised metal illness then it could be argued that the individual had suffered a detriment due to his illness. I think its worth looking into it further, go to CAB, they will be able to advise you properly on your individual case.