Apologies for not getting back to you sooner, I experienced some temporary connection issues and could not get back on the site until now. All appears to be resolved now so I can continue dealing with your query.
As far as the law stands it is not illegal to secretly record a conversation between two people. For example this could happen often in situations of bullying where the bullied individual make a recording to use as evidence when making a complaint. Obviously the bully would not have given their consent to being recorded and most likely would not have known that he was being recorded. However, such recordings can be used as evidence and they are legal to make.
There wouldn’t be a data protection issue unless personal information about an individual was being recorded. So any normal conversation would not be covered by data protection principles but if specific personal details like address, date of birth, etc were recorded then these could be covered. Even then the recording could be amended to remove that information and it will no longer be infringing.
You are free to make your own counter-complaint against that employee and the employer could hear it but you cannot prevent the employer from using the recording if they wanted to.
I'm sorry if this is not necessarily the answer you were hoping for, however I do have a duty to be honest and explain the law as it actually stands. This does mean delivering bad news from time to time. I hope you understand and would be happy to provide any further clarification if needed. If you are still satisfied with the level of service you have received I would be grateful if you could please take a second to leave a positive rating by selecting 3, 4 or 5 starts at the top of the page. Thank you