Etiquette Analysis: On Criticism
While the following Emily Post quote is geared specifically towards husband and wife, I think the sentiment is applicable to everyone. Keep your criticisms private.
“Nor does a gentlemen ever criticise the behavior of a wife whose conduct is scandalous. What he says to her in the privacy of their own apartment is no one’s affair but his own, but he must never treat her with disrespect before their children, or a servant or anyone.” Emily Post (1945). Etiquette, 587.
Public criticism often leads to public embarrassment and I personally believe embarrassment lasts a lot longer than most things. Likely, your criticism can wait to be shared with that person at another time so bite your tongue and wait for a more opportune moment where your CONSTRUCTIVE criticism can be heard without the shadow of shame. If your criticism is not constructive, I suggest you forever bite your tongue on the matter.