Truth is an absolute defense. Was your ex partner abusing the animals? If so, why didn't you report the abuse to the authorities? Know that people sue other people every day, even when they do not have a good case. I hope you carry business insurance. Broad form general liability insurance is good.
I wholly agree with my colleague. Further, always couch whatever you state as "In my opinion..", as opinion statements are generally not actionable. It doesn't necessarily mean that he won't sue you, but as to defamation, you will be fine. The only other thing that you must watch out for is not to take those clients going to him if you know for sure they are his clients -- that is actionable.