I don’t fully understand your situation as a new Strata plan and bylaws can only be registered with the State once a Special Resolution has been passed. If this has not happened then anything this owner has done is illegal damage to common property and you can ask them to return the common property to it’s original state.
The only time the OC would be liable would be if a previous owner did the faulty work and then the OC would then be liable for the cost of returning the common property to it’s original state (and not repairing the faulty structure).
The OC is never responsible for maintenance of alterations done by owners unless it specifically states that is the case in the bylaw added at the time of the request for the change and registered with the state gov.