The first thing you'll want to do is read up on Accounts and Privileges in Filemaker Help. You'll also likely find it helpful to click the Advanced search link above and search under the terms "record locking". For examples on how to use accounts and privileges to control what records a user can see and what actions they are permitted to take.
It is possible, using those settings to control which records a specific client can see and/or edit. A client can be restricted to just "their" records and your employee, logging in with a different password and privilege set, will be able to access all records.