Is one who witnessed adultery and denied it in order to conceal the perpetrators sinful?

One who conceals the faults and slips of Muslims does not commit a sin, according to the words of the Prophet, peace and blessings be upon him: "Whoever conceals a Muslim, Allah will conceal him on the Day of Resurrection." And according to his words to Hazzal: "Had you concealed him with your garment, it would have been better for you." It is preferable to use allusions (ta'areed) instead of outright lies. Therefore, one should advise the disobedient and instill the fear of Allah in them. If they do not desist, one should threaten to expose them and inform someone who can prevent them, without explicitly stating adultery. Rather, one should say: "I saw so-and-so in a situation that angers Allah."