Laws of Lashon Hara /5 : 1 (47)

1. One is forbidden to speak about a person who refused to do him a favor, even if he knows that this person was able to help him but refused to do so the speaker transgresses the commandment ‘Do not bear a grudge’, and at times also ‘Do not take revenge’]. This is so even when the speaker was not the one affected by this act, therefore it is forbidden for Levi to speak about how Reuven did not help Shimon.