Many conversations can be productive without an explicit argument being made. For example, there are many ways to deal with the various claims that God is the source of morality. Many times it is enough to paint a clear picture of what the theist is actually claiming to believe.
For example, if the person is claiming that we need the Bible as God's word to know what is moral and what is immoral, ask them to indulge you for a minute.
Say the following:
Let's imagine that we go back to a time when one of the books of the Old Testament is being written.
How about specifically:
If the offering is a sacrifice of well-being, if you offer an animal of the herd, whether male or female, you shall offer one without blemish before the Lord. You shall lay your hand on the head of the offering and slaughter it at the entrance of the tent of meeting; and Aaron’s sons the priests shall dash the blood against all sides of the altar. You shall offer from the sacrifice of well-being, as an offering by fire to the Lord, the fat that covers the entrails and all the fat that is around the entrails; the two kidneys with the fat that is on them at the loins, and the appendage of the liver, which he shall remove with the kidneys. Then Aaron’s sons shall turn these into smoke on the altar, with the burnt-offering that is on the wood on the fire, as an offering by fire of pleasing odor to the Lord.
(Part of this exercise is to display the scene for what it is.)
Imagine we are standing behind the priest and we watch him take a goat from the herd and cut its neck open with a knife at the entrance of the tent. Other priests help take the blood that's flowing and dash the altar on all sides, making it all kinds of bloody. The priest is now ripping into the body cavity and removing the fat from the intestines of the goat. Now he's takes out the two kidneys, removes the fat from them and puts them into the fire. The priest turns and talks to us and says, "You see, we do this because God likes the smell."
We're watching this scene together, and you point to this man and say, 'Without this guy telling us what God thinks is moral, how would we ever know what was good or bad?'"
Indeed. Without this priest writing down what God likes to smell, we would be lost. We would be equally lost if we didn't have him telling us what God thinks is moral.