Without confirming any such event you cannot take any precautionary measure nor you can take any legal action agaisnt a non existent person.
It is a psychological fear or an imaginary notion due to fear psychosis
psychosis means a loss of contact with reality; it is a symptom of a number of mental illnesses rather than a medical condition in its own right.
You can refer her to a psychiatrist or an experienced clinical psychologist.
If at all you confirm that somebody is creating nuisance to her as apprehended by her, you may first issue a legal notice instructing him to refrain from such an indulgence, failing to respond or cooperate you may proceed through criminal law through police