Very great question
I also want to commend you for having a conversation with an atheist.
I hope every discussion God uses you to bring your friends to Him.
I would like to share my response to this subject.
Why do we need a Moral law giver?
You know, when someone raises the question of evil, someone has to assumes there is good. When someone assumes there is good, someone assumes a moral law giver, an objective point of reference to which we differentiate good and evil.
And the moment we come to the moral law, we must posit a Moral Law Giver.
But why? Why do we need such Moral law giver?
Here are my 2 lines of thoughts.
1.) Every time someone raises the question of evil, it is either raise BY a person or ABOUT a person.
We can’t raise the question of evil without positing the value of human life.
Life is sacred, it has value, therefore the question of evil ends up to the very Person of God who gives value and sacredness of life.
Our lives has no value if we are just a product of time + matter + chance.
We come to the Moral Law Giver because the question of evil is either raise by a person or about a person.
2.) An objective point of reference, which is the moral law must come from an objective Moral Law Giver.
We as subjective human beings cannot create an objective moral law, only an objective Moral Law Giver, who is God, can posit an unchanging point of reference.
I hope this simple answers help you.
Thank you for the time to read this.