Avoiding accessing unauthorized data - architectural level

I have one SQL Server database that will contain data for multiple customers each customer has admin users to manage his own data related to his employees, I will give each customer a license to use the system to store and manage his business data ( data related to his employees) each employee has an account and has some privileges to do some specific actions and request.

My question, How to permanently prevent customer admin users to access employees of another customer? I am asking about the best practices or Ideas related to the system architecture level to avoid this kind of errors, I don't want to leave the decision to the developer because any wrong SQL query will lead to this issue.