At one time the DCIM folder was indeed the default, central location to store photos, but Android has devolved since that time and now some developers have opted to use any number of different folders to store photos in a phone. You're now likely to have a mix of different folders to check -- DCIM, Photos, Pics, images are used by most (but not all) image-relative apps, while a Downloads folder is often the default with web browser and text messaging apps.
Keep in mind that a photo manager app on her phone, like the Gallery or the Google Photos app, will display her entire photo library as one cohesive collection even though some photos are actually stored in different folders and locations.
I'd recommend you locate and clean up her collection of photos using a file manager app on her phone instead of trying to do the same thing remotely using your Mac. Samsung includes a basic file manager app on its devices, look in her Apps menu for the aptly named 'My Files' app. The My Files app has a nice user-friendly interface as its limited feature set is more than adequate for most users. Using an actual, native app on her phone to locate and rearrange things first might work out better. Then focus on transferring things to your Mac.