回复：Should've, could've... Don't panick!

One argument could be:
Since she didn't timely insist her rights to the kittens upon the first call for help, the buyer deemed the silence as consent that buyer has full rights to deal with the kittens, and the buyer already took actions fully relying on the deemed acquiescence (including posting ads online and found new owners of the kittens). This can be regarded as promissory estoppel even in the absence of written consent, and thus, the breeder lost her rights to the kittens