Matt – many Special By-Laws (SBL) that permit the keeping of pets stipulate that they’re not to be on the Common Property at all unless they’re restrained and/or being held by their Owners.
So perhaps ask your friend to in the first instance check compliance by both her and the other party with the provisions of that SBL and to then, in the light of that, consider whether any legal action could be taken or be justified in the circumstances, and if so by what process that might best be commenced – such as with the assistance of Community Justice Centre (in NSW) if she considers that the other party has indeed breached that SBL.