What is the ruling on dealing with non-Muslim customers and those who commit sins, and what is required to be done towards those whose harm one fears? Is there sin in remaining silent about an evil due to fear or harm to one's business?

Changing evil has degrees: by hand, and if one cannot, then by tongue, and if one cannot, then by heart, and that is the weakest of faith. Your duty is to repel evil and forbid it according to your ability, and if you fear for yourselves, then it suffices to detest it in your hearts. The general principle is that a Muslim should be prepared to bear the harm resulting from enjoining good and forbidding evil. Scholars have unanimously agreed that enjoining good and forbidding evil is a communal obligation (fard kifaaya); if a person fears for his life or property, then forbidding by hand is waived, and detesting it with the heart becomes obligatory. Inform those in authority if you see a benefit and are safe from harm. Do not abandon forbidding evil for fear of your trade or loss of customers, for Allah is the Provider, and piety increases sustenance.