Hello RYF_JR210,
If I understand this correctly, when you're mentioning "linked to his dads account", it is referring to his dad's Apple ID is signed in on the device am I correct?
If that is so, the first thing that your friend needs to do is to sign out of the Apple ID via "Settings > [Name] > Sign Out"
Upon signing out he will be prompted for the Apple ID password if the Find my Device feature is on, if not it will sign the account out of the device and will in turn ask you whether you want to keep a copy of the data on the device or delete it.
This whole process will not affect the data on his dad's device.
Hope that helps in clarifying.
All the best!