The only one I used a "friend" on was Chicago. Here is what they state:
Chicago Booth Application Instructions
The second letter of recommendation can also be professional but it is not required. If you would like to provide a recommendation that extends beyond your work environment you are welcome to do so. Examples may include but are not limited to a mentor, a professor, or a person you work closely with in a volunteer capacity. These letters can give us a different perspective of your skill sets outside of your professional environment. There is no preference on who supplies your second letter of recommendation our only guideline is it should add new and valuable insights to your qualifications.
Whomever you choose to write your recommendation make sure they know you well and can offer specific examples of your performance and contributions to the organization.
Also, I am curious who you used for your second recommendation in your apps, considering your claim that coworkers should not be used. Do you have two current direct supervisors? Did you pull a professor from 5 years ago? A former supervisor from 4?
The only app I used my direct supervisor for was Wharton, and that didn't really pan out. With colleagues as recommenders, I am 2 for 2 for interviews.
I work in advisory (consulting/audit) so I have at least a dozen "supervisors" who could technically write recommendations - every engagement I work on has a different manager. I'm surprised recommendations from colleagues are accepted.. I guess it worked out well in your case!