Can an honest review of a person's business dealings become actionable as slander?

Defamation (slander is a form of defamation) is when someone publically makes an untrue factual statement or assertion about another, which damages the other's reputation.

What you consider an "honest" review can be slander if you made a factual assertion which may not be true.

For example: you wrote in your review that she lied to you. Saying that someone lied is a factual statement. But suppose she did not lie? If she said "not to worry," for example, that is not the same thing as saying "shipping doesn't cause any health issues." To use an analogy, someone could tell you to not worry about playing basketball during a thunderstorm, because lightning hardly ever hits basketball poles; however, that statement, while true, is not the same thing as saying that lighting *can't* hit basketball poles. Similarly, this breeder could have told you to not worry because the chance of shipping making the cat ill is very slight--but that is not the same thing as saying the shippping does not or cannot cause health issues.

So depending on what exactly she said and what you accused her of saying, it is possible that you slandered her by accusing her of a lie when she did not in fact actually lie, since she did not say the precise thing you accused her of.

And yes, you can slander a person who has breached a contract or violated a warranty--their wrong does not allow you to defame them. All that matters is what they actually did or said, and what you accused them of doing or saying.