In the type of case described it is the owner who would seek Council approval, but the OC would have had to approve the application to Council, i.e. it needs the OC seal (am assuming this is NSW).
So it is unlikely the Council approved the works. But you can check by having a look at Council records for the property.
Jimmy is right – the OC can require the owner to reinstate the balcony, so I think the easiest route is for the owner to agree to a by-law. That leaves the matter of Council approval, and if I were the OC I would be requiring that as well.