Here is a bit of context:
The computer is not shared with anyone. The name of the Windows session is obviously not "JPDenis". We know no one bearing this name. The Window session as a password that is not the strongest but not the weakest either. This file was not edited (or stored) on another computer. This computer was mainly used inside his home, with a wifi network that as a good key, behind the internet provider's box. I personally installed a fresh Windows 10 with a genuine ISO about 10 month ago. There is no antivirus on the computer, apart from "Windows Defender" (or something like that). No craky software is installed on the computer. Windows is up to date. I am quite sure I installed LibreOffice from a file downloaded at the official website. The browser is Firefox with almost no addons. My friend is not a tech savy and installed very little software on the computer and none that I think could be harmful. He knows that suspiciously looking emails should not be opened.
My friend is quite upset about this strange thing. I will eventually Linux his computer but we would like to understand what could have happened.