Well, depending on the drive and if this 40gb of loss was one file and how quickly he shut down after saving the data, it actually is possible. Some drives will use write cache to speed up the perception of the write speeds of a drive where new data is very quickly written to a faster cache, then is transferred a little more slowly to permanent storage. But this write cache isn't always power loss protected. If you do a normal shut down, the computer waits until any data on the write cache gets transferred to permanent storage before it fully shuts down. If you just nix the power though, that data could be gone, and if it was part of a larger file, it would corrupt that file.

Edit: Here's a source to back up my info. Though, it looks like it may actually be an os feature instead of a drive feature. https://www.iolosystem.com/resources/disk-write-caching.html

This is incorrect too. The OS buffers writes to drives for performance, a portion is kept in memory and flushed to disk when possible. A sudden power loss can easily result in a partial write.

That said a drive failure is also possible.

The filesystems journal will be the source of truth.

Mem cache is definitely still a thing. Non-volatile storage has gotten faster in recent years but it’s still not as fast as RAM. Depending how his system is configured, data loss is definitely possible.

That being said, unless it was one big file and he lost critical data that made the file readable (say an MBR on a disk image) there’s no way he should have lost 44GB.

It actually might be, because now a magic file appeared on my hard drive (a file that was once deleted) that I can't delete. When I try to delete it, even with admin rights, it says No such file or directory