Mojo – with regard to the O/C selling part of its Common Property to the Owner, that requires a special resolution with ≥75% voting in favour in order for the matter to pass, and so far as the price (as opposed to the value) is concerned, it’s really up to the Owners Corporation (O/C) to determine, provided the price has some basis that’s defensible.
One further string to the O/C’s bow may be that, to the best of my knowledge, the installation of a grease arrestor requires the prior consent of Sydney Water, by way of an Application for “Commercial Trade Waste Permit” by the Owner of the property. So a representative of the O/C should perhaps contact Sydney Water with the relevant details of the property (address & lot number) and ascertain who the “applicant” was, who it should have been assuming a Rep. of the O/C (such as the Strata Manager) didn’t sign an Application, the status of any Permit that’s been issued, and to who its been issued.
Finally, even if the original resolution for the Owner to alter the Common Property in the manner that you describe was properly made (i.e. by special resolution), then the O/C may at a later General Meeting vote in that same way to rescind its earlier decision. The Owner concerned could seek to challenge that decision on the basis that she’s already acted upon the original resolution, but in the circumstances that you’ve outlined with respect to this entire saga, I doubt such a challenge would succeed, particularly with the O/C obtaining good legal advice beforehand.