Is Morality Determined by Society?

Questions about how to respond to someone who says morality is determined by society, whether our evolutionary biology causes us to think it’s objectively wrong to torture babies for fun, and whether someone with multiple personality disorder could both trust and reject Christ.  

• How should I respond to someone who is unpersuaded by the moral argument for God, who insists people just do what makes them happy and doesn’t offend people or the law, and who says morality is determined by social structures that have evolved?
• Can the idea that it’s objectively wrong to torture babies for fun actually be explained by our evolutionary biology making us want to protect the babies in our group?
• Would it be possible for someone with multiple personality disorder to trust Christ with one personality and reject him with another? If so, how would we make sense of their eternal state?