It's not clear how a slow leak from a toilet gasket could remain undetectable until there was a 7' radius of plaster damage to the roof below the toilet. Nobody looked up?
As a general rule, liability would flow from negligence -- that is, if the owner or occupant of the condo did not know about, and could not reasonably have known about, the leak, they would not be responsible for the damage. It's possible that the condo owner's duties are modified by the CC&R's. There may also be an issue of negligence in relation to the installation of the toilet.
Ideally, all parties will be insured and they'll let their insurance companies figure things out.

