All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallahu ‘alayhi wa sallam, is His slave and Messenger.
The person who praises another for a sin that he committed is sinful. He is misguided and he misguides others, and his praise may encourage the other to commit that sin again, and thus he would be cooperating with him in sin and aggression. Allah, The Exalted, says (what means): {And cooperate in righteousness and piety, but do not cooperate in sin and aggression. And fear Allah; indeed, Allah is severe in penalty.} [Quran 5:2]
Accordingly, he may burden himself with the sins of others as well, and not merely with his own sins. Allah, The Exalted, says (what means): {That they may bear their own burdens in full on the Day of Resurrection and some of the burdens of those whom they misguide without knowledge. Unquestionably, evil is that which they bear.} [Quran 16:25]
Moreover, Abu Hurayrah, may Allah be pleased with him, reported that the Prophet, sallallahu ‘alayhi wa sallam, said, “He who called (people) to error, he shall have to carry (the burden) of the sin, like those who committed it, without their sins being diminished in any respect.” [Muslim]
However, saying such statements is not disbelief unless the sayer believes that such sins are lawful; please, refer to fatwas 91110 and 258232.
It should be noted that the Muslim must be mindful of every word and statement that he utters lest he utters a word that incurs the wrath and punishment of Allah, and then the consequent outcome would be regret. Allah, The Exalted, says (what means): {Man does not utter any word except that with him is an observer prepared (to record).} [Quran 50:18]
Abu Hurayrah, may Allah be pleased with him, narrated that the Prophet, sallallahu ‘alayhi wa sallam, said, “A person utters a word thoughtlessly (i.e., without thinking about its being good or not), and as a result of this, he will fall into the fire of Hell deeper than the distance between the east and the west.” [Al-Bukhari and Muslim]
Allah knows best.