If it were me, I would alert the breeder, but try to do it in a non-accusatory way. Like, "Hey, I noticed this ad for a couple of the cats you bred, and I thought maybe you would want to contact the owner and work something out." Then I would just leave it at that, and let the breeder decide how to proceed.
The thing is, there is no reasonable expectation of privacy for the owner here. She posted the information on a public site. Anyone can see Kijiji. The breeder might have stumbled across it herself. So you aren't spilling any secrets here. Nothing was told to you in confidence.
Cats can find great homes through sites like Kijiji and Craigslist, but it's not a guarantee. If the contract isn't being upheld, I think the breeder has the right to know that. If she decides to just let it go - and she very well might - then fair enough, but at least no one pulled a fast one on her.
The Boys: Maisie, Zephyr, and Cullen.