Who is right? Both are, probably. The statement “you cannot be good without God” does not mean what the atheist, or non-atheistic member of the public, think it does.
Philosophers ask: Can people be (objectively) good without (the existence of) God?
The atheist answer: People can be good (according to society’s majority opinion of good) without (believing in) God.
The atheist gives a red herring answer: “Without God” and “without believing in God” are not synonymous.
The real question, for better minds than mine, is: “If no higher being exists, is their anything/ anyone that can objectively be called good? Or would that mean the sadist’s subjective standards of good is as valid as the standards of the philantrophist, because no arbiter higher than the sadist and philantrophist can say which standard is right?”
You cannot answer that one by being a respectable atheist. But then, the atheist who understands the question don’t have to feel offended. This is no more a slur on the atheist than on the believer: If no higher being exist (so say this idea), neither the (falsely believing) religious nor the atheists can be objectively good.