What version of Filemaker? With old versions of FileMaker, you could take a copy of the file, use a third party "password cracker" program to force the copy open. Then you could pull up and read all the passwords defined in the file. You could then use these passwords to open the original, unmodified copy of the file.
In newer versions, you can't read the defined passwords when you go to Manage | Security even if you force the file open with such a utility--which typically modifies the original file in ways that may threaten it's future integrity.
Your best bet is for the company to track down the former employee and demand the password. I'm no lawyer, but suspect that the company is completely within their rights to threaten legal action if the former employee does not hand over the full access password.