Working with friends can be fun. But to keep these relationships smooth, you need to follow a few rules. First and foremost, never send a business email disguised as a friendly "hey, how are you?" Conversely, if you're reaching out as a friend, don't talk shop. Maintain a healthy separation between business and your personal lives. When you do need to reach out to a friend about business, do it under separate cover. Send a friendly email asking about her new job and her kids and give her a heads up that you are going to contact her separately about the proposal you discussed. That way, she can forward your professional message onto colleagues without all of the personal details in it. It's important to be both sociable and professional. But with friends, keep those things distinct.
Source: "Stop Mixing Business with Pleasure" by Jodi Glickman