What is the ruling of Islamic law on buying a slaughtered animal from a Muslim who does not pray and eats pork, when a religiously permissible alternative is available?

It is not permissible to eat the meat of an animal slaughtered by a person who does not pray at all, because such a person is a disbeliever (kafir), and their slaughtered animal is considered carrion. As for eating pork, it is forbidden by scholarly consensus, and whoever does so is sinful, but they do not become a disbeliever unless they consider its consumption permissible.