A person was smoking in the room and I saw him doing thatNext day another person asks me what he was doing yesterday in that room If I say he was smoking then it's geebah How to answer these kind of questions

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 is His slave and Messenger.

If that smoker conceals his smoking and does not do it openly, then do not mention it to anyone; cover up his fault, and do not tell the one who asked you that he was smoking, because this is backbiting, or you can apologize for not answering or say to the one who asks you “Allah knows best.”

But if the smoker does it openly and does not hide it, then there is nothing wrong with you telling the one who asks that he smokes, which is not considered a prohibited backbiting.
Ibn Rajab, from the Hanbali School of jurisprudence, said in Sharh al-Arba’een An-Nawawiyyah:

“You should know that people are of two types: First one: A person who is not known for committing sins; so, if such a person made a mistake or sin, then it is not permissible to reveal or speak about it because that is forbidden backbiting. The second type: A person who is famous for committing sins and does them openly, and does not care about what he committed, nor what is said to him; such a person is immoral, and speaking about him is not considered backbiting, as stated by Al-Hasan Al-Basri and others.” [End of quote]

Al-Ghazali said in Ihyaa' Uloom ad-Deen when he mentioned the immoral person who commits sins openly: “If you mention what he does publicly, there is no sin on you. The Messenger of Allah, sallallahu ‘alayhi wa sallam, said: “He who throws the veil of modesty away from his face, there is no backbiting in speaking about him.” ‘Umar said: “There is no sanctity for the immoral person.” He meant the person who commits sins publicly, unlike he who conceals them. The sanctity of the one who covers up his faults and sins must be considered.”

As-salt ibn Tarif said, “I said to Al-Hassan: ‘If I mentioned an immoral person who commits immoralities publicly, with something he does, is it backbiting.’ He said: “No.” [End of quote]

For more benefit on when backbiting is permitted, please refer to Fatwa 82663.

Allah knows best.