ANSWER:

If he such a person was labeled as kafir for not praying, by a Muslim court or authorized Islamic center, you can’t eat from what that person has slaughtered because he is a Kafir. But if it was not proven that he is an apostate, you can eat from it. 

QUESTION: I learnt that if a person abandons salah then we should not stop replying his salam cz conditions re fulfilled or not,we don’t know yet.If we stop replying his salam that indirectly means we takfir him kafir.Q is, then why are we prohibited to eat slaughtering of those who doesn’t pray?In their case, conditions are fulfilled or not,we also don’t know,not eating their meat,doesn’t it do takfir