I am sorry to hear from your father.
The administrator password can be easily reset to a Mac if you have the correct installation disc that matches the version of software running on the computer. Resetting the password does not clear any information on the Mac. However, you will not be able to access other passwords stored in the system for email accounts or wireless keys that were previously entered and saved by the original user.
Once you have changed the password, you will be asked to create a new keychain. This is the name of the place where these other passwords are stored.
Once you have administrator rights, you can create or delete accounts.
Visit the Apple Web site for a password-reset support article: http://support.apple.com/kb/HT1274
If you do not have the installation CD or need further assistance, we recommend that you book an appointment at the Genius bar in your Apple Store so you can reset it for free.