While you delete a volume on OnTap 8.3 and above it will go to VRQ (Volume Recovery Queue)
Deleted volumes will be retained in a recovery queue for at least 12 hours before being completely destroyed.
This feature is added on OnTap 8.3 to provides recovery capability for accidentally deleted flexible volumes
To permanently delete the volume without waiting the default 12 hours, purge the volume use follwing command.(in diag level)
::*> volume recovery-queue purge -vserver svm_name -volume vol_name
Before you delete the volume, its possible to rehost it to another SVM (in OnTap 9)
Using the volume rehost command.
Once you delete the volume from any SVM, there is *no way* you can access the data in another SVM.
Even though OnTap will not technically erase each block, it will marked them as empty block.
which mean new data can be written there.
If your customer is more conserned about data security, you can suggest using Ontap9 and make use of NVE (NetApp Volume Encryption)
You can find some info related to NVE here