1. You can look for a script that was created by David Litchfield of Next Generation Security Software. He came up with a one line command app that will do an attack to determine the pwd.
2. If there is a PC or laptop that the old admin used to use, you can try Enterprise Manager Passview. This will only work if the admin did not ask for the login prompt to be shown every time you connect. find it here: http://www.snapfiles.com/get/emp.html
3. Another resource for passwords (securing and cracking them) : http://is-it-true.org/pt/ptips12.shtml
The management needs to also look at legal avenues, as this is highly illegal and the former admin could be jailed/fined for his reluctance to cooperate with the company.
Best of luck