Is it obligatory to attend the funeral of someone who bore witness that there is no god but Allah, but never fasted or prayed and always drank alcohol?

Attending a Muslim's funeral (washing, shrouding, praying over, or burying the deceased) is a communal obligation (fard kifaya). If a sufficient number of people perform it, the obligation falls away from the rest. One who abandons prayer is, according to the soundest opinion of scholars, a disbeliever. Therefore, it is not permissible to pray over them or bury them in Muslim cemeteries, based on the Almighty's saying: "And never pray over any of them who has died, nor stand by his grave. Indeed, they disbelieved in Allah and His Messenger and died while they were defiantly disobedient," and "It is not for the Prophet and those who have believed to ask forgiveness for the polytheists." As for a disobedient Muslim who maintains prayer, prayers should be offered over them.