If that is what his employment agreement states, then he has answered your own question. I can see google making a case that he "used" code from google or some

If that is what his employment agreement states, then he has answered your own question. I can see google making a case that he "used" code from google or something; remember it doesn't have to be true to take you to court or bog you down for months and thousands of dollars. For google lawyers it is signing few sheets of paper, for you it'll be putting your life on hold. Best to resolve this before you start IMO.