mbaker01:
There is no employment law or privacy issue to keep the new manager from reviewing the files of the employees he manages.
I agree that there is no specific law barring this but I disagree with the inference that it is therefore OK. There are substantial risks depending on what's in the file and what the supervisor does with it. A "need to know basis" is considerably safer.
Employee files often contain information that the supervisor has no need to see, some of which can give rise to discrimination claims.
For example, if your organization happens to keep I-9s in the EE file (not recommended but certainly not illegal), then the supervisor would have access to age and national origin. The W-4 may indicate marital status and the worksheet that is generally never removed but sometimes filled out may indicate how many children they have. The manager has no specific right to have or general need to know any employee's home address (ever read a stalking suit filed by an employee on an agent of the employer?)
I would want to know what the supervisor wants to know and then tell them if they have a legitimate business interest in that information.
Also, it is important to note that often line supervisors and management employees, while having similar responsibilities as agents of the company under a number of statutes, are not really equal in terms of their training and experience, and thereby their sophistication and judgment in handling information in employee files. There are reasons why we don't let every bozo who falls under the management definition in the NLRA look in employee files.