I don't think it's a PWA if she was using Firefox, because Firefox removed the little PWA functionality their desktop browser offered a while ago (this is one of the reasons I still have Chromium installed).
I imagine during the process, WhatsApp opened an ms-store: link that launched the Microsoft Store, and not knowing better, they clicked "install" when prompted.
The desktop app has some features that the web browser version lacks, like video calling support, so I would argue the desktop app is probably what you would want to use as a WhatsApp user, but it's rather annoying that web apps are pushing so hard for people to install desktop applications when their web apps could have the same features if they bothered implementing them in a non-Electron environment.
I just checked it, it opens a small Microsoft Store window with button "Get". In other languages they also used shady terminology to replace word "Install".
I imagine during the process, WhatsApp opened an ms-store: link that launched the Microsoft Store, and not knowing better, they clicked "install" when prompted.
The desktop app has some features that the web browser version lacks, like video calling support, so I would argue the desktop app is probably what you would want to use as a WhatsApp user, but it's rather annoying that web apps are pushing so hard for people to install desktop applications when their web apps could have the same features if they bothered implementing them in a non-Electron environment.