The “moral basis” argument (if one wishes to call it an argument) is an exceptionally feeble one which is sometimes used to validate the seemingly repulsive code happily presented as “Divine Law” by the semi-literate followers of Abraham. One needs not even answer the ludicrous question: “If you don’t believe in God, where is your moral basis?”
This is a difficult question to be sure and one which most people could not answer without an advanced degree in philosophy. The lay-person’s simplest response (and the most penetrating ) would be to simply specify where the humanist’s moral basis is not: the seventh-century and prior. That should be enough to at least keep the discussion going.