I am sorry to hear about your father.
It is possible to reset the administrator password on a Mac quite easily, provided that you have the proper installation disk, corresponding to the version of the software running on the computer. Resetting the password does not delete any information on the Mac, but prevents you from accessing other system-saved passwords for items such as email accounts or wifi keys previously. entered and saved by the original user. .
Once you have changed the password, you will be prompted to create a new keychain, which is the name of the location where these other passwords are saved.
Once you have administrator access, you can create or delete accounts.
On the Apple website, you will find a support article on how to reset the password at the following address: http://support.apple.com/kb/HT1274.
If you do not have the installation disc or if you need additional help, I suggest you make an appointment at the Genius Bar at your local Apple Store. They will help you reset it at no cost.