Thank for asking to HCM
I really appreciate your concern for your niece, this could be post operative event after the cardiac surgery
in valvular heart disease
such cardiac arrest
is very likely although this happens very rarely but chances of this condition can be neglected, is she remained unconscious for a long time or breathless then the cerebral damage would be there, this is real medical situation, you need to talk to medical team, take care and sorry for the condition.