Help! All data seems lost despite Time Machine backup.

I'm trying to help someone remotely. They have a WD external drive which has always shown in Finder as two partitions: 'WD Unlocker' and "Time Machine". He's been scrupulously making Time Machine backups to the "Time Machine" partition for years.

Due to corruption on the internal drive of his Mac, he erased that drive from Recovery, reinstalled Big Sur, and intended to migrate the data back from the Time Machine backup.

But here's the problem: only the "WD Unlocker" icon appeared on the external drive, and he could not migrate his data because the Time Machine partition was not accessible. Prior to this, the external drive always unlocked automatically, revealing the Time Machine partition.

When double-clicking on the WD Unlocker, he gets the message: "You must run the WD Drive unlock application from the WD drive unlock CD associated with the drive you want to unlock." But the original CD is unavailable, and, in any event, there's no CD drive on the Mac.

He then tried to revert to the point at which the most recent backup was made by choosing "Restore from Time Machine" instead and got the same problem as the Time Machine partition was not accessible.

So how can he restore his data from the Time Machine backup?

Also, can the WD Drive unlock application that used to be provided on a CD be downloaded from the Internet?