In Nemo, I go to .wine/drive_c. I look in Program Files: not there. I look in Program Files (x86): not there either. Doh! I see portapps has created its own folder on drive_c top level, and inside that /whatsapp-portable.
There's a readme.md (nothing useful in it), a "whatsapp-portable.exe", changelog, portapp.json and a folder called "app". Looking in /app, more folders and "WhatsApp.exe". And in one of those folders, another "WhatsApp.exe".
OK, I'll try the whatsapp-portable.exe in /whatsapp-portable and see what happens.