If you have a disagreement with an employee over when their probation period ends, here's how to figure out the situation. Check the contract.
Employers often rely on shorter notice periods during probation when terminating an employee's contract, as this means paying less notice pay. However, the strength of the case will depend on what has been explicitly included in the contract of employment. Just because you haven't officially confirmed a successful probation period, don't assume that your employee will still be 'under probation' by default.
For example, if a contract simply states that the first 3 months of employment will be a probation period, during which a shorter notice period of 1 week applies, and an employer tries to terminate the employment on the first day of the fourth month, then the employee is probably entitled to notice pay of more than 1 week.
However, if the contract includes a statement that the probation period is not considered to be successfully completed until this has been confirmed in writing to the employee, an employer could reasonably rely on this to dismiss the employee according to the terms of the probation period.
This is just one example of why it's important to get your contract right in the first place, for more information see this article with handy tips for producing an employment contract.