Marcus Aurelius words have no authority but are very poetic and are a not-unreasonable position for an agnostic to consider. I don’t say ‘atheist’ who presumes to say “God does not exist” as if ‘atheists’ know for certain something of which they cannot truly be certain. They can only honestly say, “I do not believe that God exists”.
However there is ambiguity in what Marcus Aurelius says, as what is a definition of a good life or a noble life? For anyone who does not accept God’s existence and authority, as you suggest, interpretations are subjective. How we apply the definitions in our own daily lives is to some degree a subjective interpretation of what God requires of our lives in the Commandments. However we agree on basic principles of what goodness means in Christian teaching and truth. Agnostics draw their interpretations from…where? For some, now, it might mean the Christian presumptions that still do permeate many parts of our world and society even where awareness of the source has been lost?
What Marcus Aurelius meant by ‘good’ and ‘noble’ is not explained in the quote…
We mean what the commandments and beatitudes teach. Jesus requires in Matthew 25 verses 31-46, practical acts of assistance and kindness to others in their needs, which Jesus reveals is loving God. We regard a good life as loving God above all and others as self.
You could ask your friend what he/she understands as good and noble, but I wouldn’t make too much of it because it is a poetic reflection from a philosophic mind not an argument against faith. It is an argument with which an agnostic might seek to reassure himself.