Since the End-user data is just stored in a separate library and IFS folder and there are multiple other End-user data on the system, there is no way to “Wipe” their data off the disk. The only way to ensure the data was gone would be to copy all of the other data to new disk (NWSSTG) and then sanitize and delete the old NWSSTG.
My suggestion would be to find out:
1. Was this a documented requirement in the End-user contract?
2. Is this a customer requirement, auditor requirement or legal requirement (PCI, HIPPA, etc..)
Depending on answer to #2, ask for a copy of the exact requirements (What standard is it based on) as there may be flexibility in what is being requested.
If the End-user is that specific about the deletion of the data, what about backup tapes your customer has?