How do you react if your co-worker is rude to customers?

That would depend on what kind of a rapport I had with him. If I felt comfortable, I would probably mention that being rude is probably going to be bad for him in the long run, since unhappy customers reflect badly on the sales associate. Otherwise, I would mention to the supervisor what I had witnessed, and let him handle it.

I fully agree with Mr Divesh Patel.

Just want to add in customer service being rude is dangerous. Even if the person being rude is a friend both of us could be loosing the job if the custromer sues the client. In the Indian context The BPO is the service provider. The client is mostly American.And if the client is sued he would  discontenue the contract which would be very dangerous for all the emploees.

So leaving freindship alone at work everyone is a profesional .