Out of the box Samsung phone's are set up to reject files from unknown sources so unless she's bypassed this in the phone's settings an apk file shouldn't have been able to be downloaded onto the phone.
Has anyone been able to have hands on with the phone which is the usual source of something not being right.
The phone has a built-in in security scanning option as well as download able apps from the Playstore to run sweeps.
A Samsung Experience Store or Samsung Service Centre can take a look @ColonelJay
And or factory reset the phone which may help but consider not using any back ups to restore information back onto the phone.
i.e start afresh.
Current Phone ~
Samsung Note 20 Ultra 5G SM-N9860_256Gb SnapDragon 865+
Samsung One Ui 3.1/ Android 11.