Can she rotate the phone 90 degrees and take a screenshot using both pointer fingers? You'd have to take into account accidentally rotating the screen so you'd have to make sure to keep it flat.
I don't have arthritis but that's how I take screenshots on phones where the buttons are all on the same side.
I'm not sure how different the interface is on OneUI, but on my Pixel when I swipe up and hold on an app to open the App Switcher, there is a button below the app that takes a screenshot of just that app.
I don't remember when it was first introduced, but it is a feature of stock Android, so it should be present in some capacity on most Android phones.
Full hand swipe takes a shot as well... like the bottom of your hand where your pinky is and swipe right. Hard to explain.
Swipe to capture: To turn on the palm swipe gesture, you'll need to go to Settings > Advanced Features > Motions and gestures and slide the switch next to Palm swipe to capture to the On position.
Aye, OP, check if you can find Goodlock on the Galaxy Store. Go into the "Life up" section on the Goodlock app and download Registrar. It should allow you to set either the back double /triple tap and/or a press and hold of the side button to take a screenshot, as shown here.
I have a Pixel 6 with the buttons on the same side. I use my left index and middle fingers to hit them. I'm sure there's some combination of fingers she could use.
I'm pretty sure the Google assistant can do it by voice, too.
If you install Goodlock (Samsung store), then Registar (under Life Up in Goodlock) then you can set tapping the back of the phone to take screenshots (or other actions)
Edit: these are Samsung official apps suite developed to customize nearly everything on your phone