The religious like to say that people can't be moral without belief in god. This is nonsense; there is not one shred of objective evidence that backs it up. In fact, empirical data suggest the opposite. There are a disproportionate number of religious people in prison and more religious societies experience greater problems with behaviors that the religious would call immoral.
If this supposed moral benefit is offered as an argument for the existence of god, then it falls in the category of arguments that are implicit admissions that the belief is a delusion. If something exists, there is no reason to argue for belief in it because the belief is useful. If it exists, then that alone is sufficient argument for belief.
If this is offered as a reason to believe, just point out that the person is admitting that religion is not really true--simply useful. If you want to be polite, you can say something like:
"So you don't actually believe in god, you believe in belief."
Usually this is not the case, however. Usually the person is using it as a setup for an argument from authority (because god is the authority we all need, we know this because his "spokesmen" have told us so on his behalf) or the person is implicitly threatening you with social disapproval--the most common stick used by believers in civilized countries to beat up on non-believers. (In other countries, they use real sticks.)
The best thing about this argument, however, from an atheist's standpoint is that it is an admission of the moral depravity of believers. If they behave morally only because they fear god, then they have no internal morality. Only small children and psychopaths have no internal morality. If someone tries this tack, simply say:
"If the only reason you behave morally is fear of punishment, then you are either a psychopath or have the mind of a small child."
If the person actually says that he or she personally behaves morally only because of fear of god, then say:
"So, you're a psychopath?"
When the person protests, point out the reasoning discussed above. Fear of punishment is not morality, it is naked self-interest. Pointing this out undercuts the religious person both on the logical level and on the emotional level--the hallmark of a good argument.
Finally, one of the best responses (credit Sam Harris) is:
"Ninety-three percent of the members of the National Academy of Science are non-believers. If what you say is true, why aren't they guilty of raping children at the same or a greater rate than priests?"