If the former IT guy "can't remember" the password I suspect you're done. Perhaps a lawyer's letter to the former IT guy would help refresh his memory. It's not credible from my point of view that a person originally charged with what seems to be a large responsibility would "forget" or fail to have recorded this password somewhere. However stranger things have happened . . . . And you also mention the owner has also forgotten the password. Perhaps simple incompetence is the issue. Either way I don't see a way of opening this file unless someone remembers the password.
At one time, FileMaker offered a service for this for a substantial fee. I don't know if they still do, but you can ask. Be prepared to prove that you have the right to access the database.
I've heard of, but never used a utility that can insert a new "admin" account into an existing FileMaker file. That would definitely be a "use at your own risk" type of widgit as it is modifying a file in ways that might not be correct and damage to the account security part of a file is very often not something that recover can fix.