Earlier quoted context omitted.
Only if the employee is informed upfront. Otherwise it's a privacy issue.
Let's assume they were informed upfront - there's nothing illegal about it then, is there?
See the link I have posted in a sibling comment: https://gdpr.report/news/2017/11/17/5383/
>The ECtHR held that the employer had breached B’s right to privacy because they didn’t inform him of the monitoring in advance and nor did they tell him that they may access the content of his communications. The previous courts had also failed to determine the reasons justifying the monitoring and whether these were proportionate to the purpose or whether the employer could have used less intrusive measures to achieve the same result.
If I read this correctly even if the person had been informed of the monitoring the evidence wouldn't have been receivable because the monitoring wasn't deemed "proportionate".