Firstly, how do you know that you've not met the person? He might be using a pseudonym on Google or he might simply be a customer whose name you've not known (or who used a different name when they patronised your business).
Finding all of someone's Youtube channels isn't particularly difficult if you know the name that they're registered with Google under. For example, if someone has a Google account under the name Freda Bloggsworth but additional Youtube channels (or 'brands' as Youtube calls them) called 'Rainham Hair Designs', 'Dagenham Car Repairs' and 'Sexy Girl in Catford', then searching Youtube for 'Freda Bloggsworth' will bring up links to videos under all of those different 'brands'.
Have you posted a response to the Google review?
https://support.google.com/business/answer/3474050?hl=en-GB