We should not say bad things about anyone, whether or not they are bodhisattvas. It is not the same thing, however, if we know that pointing out someone’s mistakes will help them to change. Generally speaking, since it is not easy to change another person, we should avoid criticism. Other people do not like to hear it and, further, laying out their faults will create problems and troubles for us. We who are supposed to be practicing the dharma should be trying to do whatever brings happiness to ourselves and others. Since faultfinding does not bring any benefit, we should carefully avoid it.
[…]
If we really want to help someone, perhaps we can say something once in a pleasant way so that the person can readily understand, “Oh yes, this is something I need to change.” However, it is better not to repeat our comments, because if we keep mentioning faults, not only will it not truly help, it will disturb others to no good effect. Therefore not mentioning the faults of others is the practice of bodhisattvas.
– 17th Karmapa
from the book “Traveling the Path of Compassion: A Commentary on The Thirty-Seven Practices of a Bodhisattva”