employers are not responsible for making their employees seek and maintain regular medical assistance for psychiatric problems (they may have to provide some services for unwell people in their employ, though) - this is down to the individual, their nearest relative and possibly the police (under a section 136) or an AMPH (a social worker or practitioner under a section 135) to assess or detain an individual under the mental health act and enforce treatment (but only in some certain circumstances).
Employers are responsible for the amount of stress their employees are subject to and if there has been an accident or a failure of the management, HR or OH systems causing that period of stress/sickness while they remain a staff member in the company - but if an employee resigns, gambles, commits a crime, then that individual is culpable, not the employer (that would be like somebody committing murder and then blaming their gp because they have depression).
however, if an employee resigns when unwell and later recants that resignation, the company may be forced to reconsider that request to terminate their employment......and they could be taken to a tribunal and sued if they did contribute to the failing health of the employee for constructive dismissal, loss of earnings and/or damages.
he responsibility for (criminal) or personally damaging acts committed outside of the workplace (however mental health issues are caused) will be on the staff member, unless it occurred during work hours or related activities (which could still be very difficult to prove) and they did not have capacity at the time. all sorts of things happen at work which cause mental health difficulties.
for example, i have been bullied by my manager in my workplace, which made me very unwell last year and i attempted to kill myself as a result. i am in the process of making a complaint against the person responsible and examining whether the employer was aware of this issue and did nothing to stop it - but will only be able to take them through the tribunal process and sue them for damages. if i had committed a murder, for example, when i was severely depressed and psychotic, my employer would not be held responsible in any way (irrespective of causing the illnesses in the first place) - that would be mine alone, as well as all the personal, financial and social repercussions of the offence. if i did not have capacity at the time i killed the person, that would be taken into account by the cps deciding whether to prosecute me, how i decided to plead (not guilty by diminished responsibility), evidence presented in a trial, and in the sentencing by the judge at the end of the process.
basically, the employee may have been treated like crap by the employer, and can attempt to sue/take to tribunal because of that and attempt to cover their losses for that process......but any criminal fallout will be that person's responsibility alone, and would be taken into account when deciding the outcome of any criminal process. whether or not a person has capacity while all this is going on is pretty secondary. no employer would be made to make right the losses of gambling or reckless spending, for example - the person you are referring to will have to sort out the employment issues and criminal stuff separately......and suck up the consequences of their actions and learn from their mistakes, like everybody else.