Just to be clear on what Family Friends are: A Family Friend is an individual who is considered to be as close as a family member but have no blood ties to the family. Family friends are usually individual(s) who have been around for many years, and may have a substantial amount of information to share about other family members that they are/were close to. Now that we are clear: The definition is provided to give you an idea of how important it is to interview Friend's of the family. They may or may not have valuable information that was shared by a family member who didn't feel comfortable sharing it when they were alive.
If you have multiple Family Friends, interview them separately, then together as a group if they been around just as long as the others. Sometimes a Family Friend can help others to jog their memory on certain events.