Can a human actually be "frightened to death?"
The people whom this happens to consider themselves average people.  

The thought is that catecholamine surge caused by the event triggers some predisposition to cardiac arrhythmia - maybe inherited, maybe due to medicines.  People predisposed to this sort of arrhythmia probably do not know that is the case, and consider themselves to be average people.  It would seem that the victims of this are not average and have some sort of predisposition to that problem.  There is a lot of interest in figuring out who there people are in advance - not in the context of being frightening to death but in the context of SUDEP - sudden death in epilepsy.  Epileptic people sometimes just die, presumably for cardiac reasons, but there is no way of knowing who among this large group is at risk. 

In the following abstract the frightening thing is being restrained.  Other circumstances that produce a catecholamine surge can do it too.  

https://www.ncbi.nlm.nih.gov/pubmed/25628339


  Med Sci Law. 2016 Apr;56(2):85-90. doi: 10.1177/0025802414568483. Epub
  2015 Jan 26. Sudden cardiac death with stress and restraint: The
  association with sudden adult death syndrome, cardiomyopathy and
  coronary artery disease.
  
  Krexi L1, Georgiou R2, Krexi D1, Sheppard MN3. Author information
  Abstract OBJECTIVE: The aim of this study was to report on sudden
  cardiac death (SCD) during or immediately after a stressful event in a
  predominately young cohort. METHODS: This study used retrospective
  non-case-controlled analysis. A total of 110 cases of SCD in relation
  to a stressful event such as altercation (45%), physical restraint
  (31%) in police custody (10%), exams/school/job stress (7.27%),
  receiving bad news (4%), or a car accident without injuries (2.73%)
  were retrospectively investigated. The majority of the subjects
  experiencing SCD were male (80.91%). The mean age was 36 ± 16 years
  (range 5-82 years). Twenty-three cases (20.91%) were psychiatric
  patients on antipsychotic medication. RESULTS: Fifty-three per cent of
  cases died with a negative autopsy and a morphologically normal heart,
  indicating sudden adult death which is linked to cardiac
  channelopathies predisposing to stress-induced SCD. Cardiomyopathy was
  found in 16 (14.5%) patients and coronary artery pathology in 19 (17%)
  patients, with atherosclerosis predominating in older patients.
  CONCLUSIONS: This study highlights SCD during psychological stress,
  mostly in young males where the sudden death occurred in the absence
  of structural heart disease. This may reflect the proarrhythmic
  potential of high catecholamines on the structurally normal heart in
  those genetically predisposed because of cardiac channelopathy.
  Structural cardiomyopathies and coronary artery disease also feature
  prominently. Cases of SCD associated with altercation and restraint
  receive mass media attention especially when police/other governmental
  bodies are involved. This study highlights the rare but important risk
  of SCD associated with psychological stress and restraint in
  morphologically normal hearts and the importance of an expert cardiac
  opinion where prolonged criminal investigations and medico-legal
  issues often ensue.