Files keep disappearing

HI, (I'm not a native English locator, so my apologies if my English is not perfect)

I use Office 365 and SharePoint with my company.

I shared a folder with a client so he can work on it, and I have access to the documents. The short way to access the file: my company name/folder1

For months it worked perfectly.

Today, my client called me and told me that his upload files keep disappearing.

After questioning him and asking him to send me a screenshot, I understood that the guy created another folder (folder_2) at the same level as folder_1. So the link to this folder is my company name/folder 2

And the files he uploaded here disappeared.

So basically, he created a folder somewhere he should not, and the files no longer exist.

- Have you heard of this problem before?

- Do you have any idea where these files could be?

It is weeks of work.

Thanks for your replies.