IMHO it is the insured who can file a complaint because it is he who has an insurance contract with the insurance company. yung injured party ay walang privity of contract with the insurance company here.
The agrieved party can lodge a complaint (even though they are not the insured) provided that they have proof that the insurance company is reneging on their obligation to pay. The firm I used to work for had two to three complaints lodged against them by an agrieved party even though the said party is not the insured. The IC entertained all complaints.