I agree with your stance on this.
However, there are some practical hurdles to overcome:
I’ll assume you just have the standard by-laws, so firstly I would suggest notifying the owners that there has been illegal work carried out to their lot, which is in breach of by-laws. I would recommend that the OC advises the owner that it will get three quotes and carry out the work itself and pass those costs back to the owner.
A balcony is common property and is usually given as exclusive use to the lot owner. As it is common property it is covered by OC insurance requirements, meaning that the OC is responsible for its upkeep, and the OC would be responsible if someone fell off the balcony.
However, all you need to prove is that OC didn’t remove the railing itself, nor did the OC grant permission to the previous lot owner to remove the railing.
Obviously the first step is to discuss the options with the owner, instruct your strata agent to follow your directions and then take it from there. It could end up in mediation/CTTT, so you need to understand from the outset how far you are willing to pursue the matter.
Good luck