One's sphere of influence - friends, family, co-workers and others - are often a significant source of clients. Working with friends can also be a source of trouble if not dealt with carefully and professionally. There may be a tendency to treat this business relationship a little less formally because the client is a friend or family member. For example, one may skip putting a detail in writing because "he's my friend, I can trust him or he will stick to his word" or falling short of a friend's expectations because the business aspect of the relationship was sealed with a handshake when you normally go to great pains to explain the agency (buyer or seller) contract.
Failure in a business relationship with friends or family could contaminate your sphere of influence affecting it as a source of business. This article, "Tips for Working With Friends", from NAR's YPN Blog provides some useful information. The author reminds that "documentation and organization are key to being successful with any type of transaction, and become even more essential when dealing with clients you have a relationship with".