The answer to your question in the title is one thing, but then there's the question within your post:
Is there any way for me to be certain of that?
For all practical purposes that question can really only be answered as a no. To be clear, you can never be 100% certain.
However, the closest you can get (without him losing all his other data*) is to ...
Not sure if this will do exactly what you want, but here's how to get a link to a file stored in iCloud:
On your Mac:
In Finder, open your iCloud Drive (found in the Go menu or Shift+Command+I)
Right click on the file and choose Share > Show People
Expand Share Options and adjust permissions as needed
Click Copy Link
On your iPhone/iPad:
Go to the Files ...