If the worker is registered for laobao, it's between the worker and the BLI (Bureau of Labor Insurance, a unit of the Ministry of Labor), though the employer is still on the hook if the salary was underreported. The penalty is the missing amount of the premiums x4, plus the missing amount of the benefit if applicable.
If the worker is not registered for laobao, the BLI can investigate and fine the employer but can't be held responsible for the insurance benefit because it's the employer's fault, so it's between the worker and the employer.
The labor department won't directly investigate a labor insurance matter because the investigation would just involve transfering the case to the BLI (or so they say), but if the department mediates a labor-management dispute it can include an insurance benefit as part of the settlement (if both parties agree).
If mediation fails, the worker needs to file a civil suit against the employer. The legal basis is the Labor Insurance Act Art. 72, and iirc the BLI's website states that Civil Code Art. 184 (tortious behavior) is also applicable if the company's registered capital is insufficient to cover the debt and it's therefore necessary to go after the fuzeren of the company.