What would you do if a co-worker were rude to customers?

First is to get the attention of the co-worker without interrupting his/her discussion with the customer. This is just to alert him/her that somebody is watching. In this way, he/she will be aware of his/her actions.

After his/her conversation with the customer, I will talk to my co-worker in private and politely asked for the reason for his/her behavior. If this is something I can address, I will give my advise on the proper way how to deal with the clients and maybe come up with a solution. Otherwise, it will be escalated to correct person with authority.

First we need to wait until our co-worker finishes with his/her talk with the customer (make sure you are too listening that conversation). Then you may speak to your co-worker what went wrong for his/her behaviour towards the customer. Understand the situation and then come up with the solution.

If you notice that it was wrong, then make your co-worker understand that it's not the correct way to behave with the customers. And then you may escalate it to your supervisor for good ending with the customer.