Hilchos Lashon Hara chapter 4, section 8

Sometimes a beis din (Jewish court) rules that a person must actively do something – whether between people or between a person and Hashem – and the person refuses to do it at all. Then it is allowed to tell about their wrongdoing, and even to write it in a book of records for future generations.

If they give excuses for not doing it, the rule is this:

If we see that the excuses are not true, and are only meant to push off the beis din, it is allowed to speak badly about them.

But if it is unclear, it is forbidden to speak badly about them.