i agree with Siddarthan Sarumathi Pandian if he has access to your source code he will have access anyway. You can ofc increase the security by only allowing access via a company account that is controlled and she/he does not know the password so she/he cannot login from home and she/he is not allowed to work with an laptop.
but as soon there is a mobile work station involved ..... you're basically as secure as using the same account.