If the Screen Time passcode is forgotten and the Apple ID used to set it is no longer accessible, here are your options:
1. Try the Apple ID recovery option built into Screen Time: go to Settings > Screen Time, tap "Change Screen Time Passcode," then tap "Forgot Passcode?" This will prompt you to sign in with the Apple ID that was used when Screen Time was originally set up. If your father can access that old Apple account, use it here to reset the passcode.
2. If the old Apple ID password is also forgotten, go to iforgot.apple.com to attempt account recovery using the email or phone number tied to that account.
3. If neither of those works, the only remaining option is to erase the iPhone and set it up as new. Go to Settings > General > Transfer or Reset iPhone > Erase All Content and Settings. This removes Screen Time along with everything else, so make sure to back up any data first.
Unfortunately, Apple does not provide a bypass for Screen Time passcodes without the associated Apple ID, as it is a security feature.