Leo says it could be as easy as going into your browser settings and change the homepage setting. If she changes that and it keeps coming back, then there's something running in the background that is running concurrently with her browser. She could try running System Restore, Malware Bytes, Windows Defender, etc. But at the end of the day, it's often best to just backup her data, reformat the hard drive, and reinstall Windows from a known, good source. Then install the latest Windows updates.