Exclusive use rights or not, the rooftop area remains Common Property and therefore the proposed balustrades would in my opinion be classified as a change or addition to that, where under Sect 65A of the NSW Strata Schemes Management Act (1996) the Owners Corporation’s (O/C) consent may only be given by way of a special resolution at a General Meeting.
Before commenting further, could you please advise by what means your O/C granted approval to your neighbours proposal?