Hi there and welcome
Can you tell em please the make and model of the phone?
Not a phone, but phones can error out and lose their information. Usually from faulty memory. Also multiple text can be part of this.
But to stalk the phone means you would have to be highly skilled with a custom rom and a lot of physical access to set this up.
Not even an advance user can do this, its really a lot of work and its hard to hide.
To me it all points to a faulting phone.
does that help clear it up for you?
Only that it is not possible to do anything without many hours on the phone and highly technical skills.
Tell her to reset the phone if she does not still believe you as this would remove anything installed.
If you would like any additional information or assistance, please do not hesitate to let me know.
Else if you can take a second to rate my service by clicking one of the stars at the top of the screen then submit, that would be greatly appreciated.