Should my father tell on visa application that he lived in the UK illegally?

My father went to the UK a long time ago on a 6 month visit visa. But, he ended up staying and working there for about 10 years. The Home Office had taken his biometrics, but in 2008, he left the UK and took a taxi to Spain. Since 2012, he has been a resident of Spain. He would now like to visit the UK again and plans to apply for a 6 month visit visa. Although it has been more than 10 years since he left the UK, should he reveal it in the application where they ask if you have ever stayed or worked in the UK illegally?