Under the Equality Act 2010, people with a mental disability are taken just as seriously as those with a physical disability, and protection is given to them against discrimination.
However, to get protection under the Equality Act, the mental health problem must be diagnosed as a disability. To be classified as disability the person must have had the mental health condition for at least 12 months and will need to be able to show that they are experiencing substantial disadvantages at work because of their illness.
Once the person has been confirmed to have a disability due to their menta health problem, it then becomes illegal for employers to discriminate against them because of their health For example, you could not refuse the persona job promotion based on the state of their mental wellbeing.
Employers are also required to make reasonable adjustment to workplace to help the person under the Act.