Rather than trying to minimize the sales rep's talk time, you may be better advised to minimize the amount of time that you are focusing on what the sales rep is doing. It is quite normal for salespeople to spend a good deal of time chatting with customers and potential customers, and although such conversations can appear to be hapless banter, they are often much more than that.
One of the most critical elements in sales is trust, and many salespeople build it by finding areas, experiences, and pursuits that they share with their customers. As a result, conversations can easily focus on non-work topics. In addition, if the salesperson's pay is heavily based on commission, then he or she has even more discretion in determining how much time to spend on calls with customers or friends.
In order to determine if the sales rep is spending too much time chatting, and too little time selling, all one has to do is look at sales figures. And, the person to do that is your manager. Speaking of your manager, perhaps it is time for him or her to do some team-building in your department so that the troops understand each other's jobs a little better, and understand each other a little better as well.
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