It is an acknowledged fact that we perceive errors in the work of others more readily than in our own. - Leonardo da Vinci

Truth at last cannot be hidden. Dissimulation is of no avail. Dissimulation is to no purpose before so great a judge. Falsehood puts on a mask. Nothing is hidden under the sun.

Surely when a man is painting a picture he ought not refuse to hear any man's opinion... Since men are able to form a true judgement as to the works of nature, how much more does it behoove us to admit that they are able to judge our faults. Therefore you should be desirous of hearing patiently the opinions of others, and consider and reflect carefully whether or no he who censures you has reason for his censure; and correct your work if you find that he is right, but if not, then let it seem that you have not understood him, or, in case he is a man whom you esteem, show him by argument why it is that he is mistaken.

Reprove your friend in secret and praise him openly.