I think the argument is interesting and partly valid. Explaining which part I like will take some explanation.

Many of our problems thinking about morality, I think, arise from a failure to make a distinction between two different things.

• Morality in daily life
• Morality as an ideal

Morality of daily life is a social convention. It serves its societal and personal (egoistically prudent) function precisely because it is a (mostly) shared convention. Almost any reasonable moral code, if common knowledge, is better than no common code.

Morality as an ideal is the morality-of-daily-life toward which moral reformers should be trying to slowly shift their societies. A wise person will interpolate their behavior between the local morality-of-daily-life and their own morality-as-an-ideal. And probably closer to the local norm than to the personal ideal.

So, with that said, I think that your Christian friend's argument is right-on wrt morality-of-daily-life. But it is inapplicable, IMHO, to morality-as-an-ideal.

ETA: I notice, after writing, that Manfred said something very similar.