There is still a misconception among HR professionals (and lawyers) out there that genuine sickness absence is a disciplinary matter and can be dealt with by way of warnings. This is clearly wrong. Disciplinary matters are based on voluntary conduct, such as any form of misconduct or performance issues based on lazyness or negligence. They can be fixed or rather addressed by a warning procedure as the employee is, in principle, capable of changing their behaviour. The warning works as a reminder of ‘better stop this or else’.
A genuine sick employee cant change their sickness. It is just there. If it cant be treated, the employee is not guilty of misconduct. It is a performance issue in the sense of the employee being capable of doing their job. Ultimately, an employee who is sick so frequently that they cant do their job properly anymore, can (or rather should) be dismissed of incompetence, but not misconduct.
Have a look here in this newsletter for another example where the employer got it wrong and issued an employee with a warning for being sick for too long:
And, related to this, here are two good examples how to treat employees with long term sickness issues: