One of the distinctions between the ethics of counseling and those of coaching addresses this very question. Licensed clinicians are taught and even legally required to not have dual relationships with their clients; this means that you cannot be a "friend" with a therapy client. In small or rural communities you may know your client through other social or business settings, but confidentiality must prevail. The therapeutic alliance also must remain uncontaminated and focused on the counselor/client relationship.
A coaching relationship, however, is often more collegial. You may coach pre-existing friends or acquaintances whom you relate with in other settings. Just as your Certified Public Accountant has a professional relationship with you, he or she may also be your friend outside of that role. Similarly, those you coach may be friends beyond your coaching role with them. I have played golf, gone out to dinner, and even gone skiing and hiking with certain coaching clients. Here is the key: You may only be friends with clients if your friendship does not interfere with the coaching of that individual. You don't HAVE to be friends with clients or interact with them outside of the coaching relationship, however. Those relationships are always a choice. Objective critical thinking about effective boundaries must always be present, and those thoughts can even be discussed with your friend/client.