I am not a lawyer but I think the OC could be liable, especially if the person who fell was a child or some other person for whom it could not be argued that they fully understood the risk. Even if the OC were not liable, it is clearly highly irresponsible.
If the OC has engaged a builder to do the work to complete the balcony, and the builder is leaving the balcony essentially unprotected, then the OC should make an urgent enquiry to make sure the builder’s liability insurance is up to date and to insist that the doors be secured. Do it in writing so the OC has some evidence that it tried to do the right thing.