Praise be to Allah.
It is not permissible to approve of those who slander the religion in this slaughter-house, even if they pray, because slandering the religion is kufr which puts a person beyond the pale of Islam. As for those who do not pray, they should not be left to do this job if they deny that it is obligatory, or they persist in not praying, or if they do not pray even at home. It is not permissible to eat the meat slaughtered by these people, even if they call themselves Muslims and mention the name of Allaah when they slaughter the meat. You should not keep quiet about them if they are in a Muslim country where the laws of sharee’ah are applied. If you cannot get them thrown out from this kind of work, then you have to warn the Muslims against eating meat slaughtered by them. And Allaah knows best.