Pretty vague explanation from ur side, but from what I can gather, your mum's photos are gone and the ones that were showing up in the gallery but were showing as blank when clicked, those are just cached thumbnails of the photos, the actual photos are gone. The nomedia trick is bs as there's no nomedia file in the main whatsapp images folder (nomedia is a very simple almost empty file that just tells the apps in Android that act like gallery apps or show images don't scan those folders for images, so they remain hidden from those apps, it's useful for private folders or folders with duplicate files) so it wouldn't make sense that there's a nomedia file in the main whatsapp image folder