Check to make sure there both on the same android version, settings>about phone> software
If they are both updated the make sure you have the battery saving/optimize features are on for both devices.
Battery usage and screen on time in my opinion is a very subjective topic as well all use different apps and set out phone's up differently with different preferences settings.
Perhaps an app the person is using is using more juice etc.
Look at the battery stats after a day's usage and look to see what's using more juice.
Consider putting the phone in Safe Mode which disables downloaded apps but leaves the stock one's working and then see what happens.
If the drain stops then it's one or more of the downloaded apps that's most likely causing this.
Start deleting one at a time until it stops.
Google Chrome as one example is a resource hog so I personally disable this and primarily use the Samsung Internet App.
A Samsung Experience Store or Samsung Service Centre can run a diagnostic test if required.
I wish you all the best with this situation.
🌈 Stay Safe & Stay Alert. 🌈
Current Phone ~
Samsung Note 20 Ultra 5G SM-N9860_256Gb SnapDragon 865+
Samsung One Ui 3.1/ Android 11.