Data Deletion Skepticism

Be thankful they did not take a copy of your hdd for forensics later.

Not publicly, as far as I know. They claim to have deleted them, but I don't think it's exactly possible to prove you've deleted every copy of digital data. You should probably assume they're compromised.

I hope he securely erased the data rather than just deleting it, otherwise it might not be just the FBI who have potentially got a copy...

realistically its probably just left on an unsecured file somewhere isn't it?

I'm sure some secret agency has backups of that data.

Data is apparently somewhat aged, so could have been exfiltrated over a period of time.

Does this actually prevent forensics from finding any data?

Perhaps assure him it's not really gone, Google, Zuckerberg, the NSA, Russia et. al. still have a copy?

I disagree. They said they have encrypted some files on the scammers computer by planting supposed banking credentials which they did copy without any consent. Having watched some similar experiences in YouTube I can say theft of banking credentials or personal data is commonplace. They probably did prevent misuse of some stolen banking or personal information.

If he took it home with him, you can't ensure the data is deleted. Could be on an external drive at home.