The question was, “Most objectivist theories of morality make no reference to God, so how can you say that only by the God of the Bible can there be objective moral values?”
Notice the word, “theories.” Theories must never be confused with reality. They are at best explanations of observations. In this case, they are even less than that.
Use active listening skills to keep the conversation on track toward an answer to this question, “How could matter, in an atheistic paradigm, account for an objective moral law?”
Just ask the question and draw out the answer. There is no answer to this question that makes sense. They may site some moral systems that don’t mention God, but that does not mean that the moral systems are objective. They may be absolutely believed, but they are arbitrary. If an unbeliever says that something is right or that something is wrong, they either must reference God or else be illogical and arbitrary.