I think what I would personally do is listen with an open mind as if it isn't your friend. Knowledge is power, and if an opportunity arises to talk to your friend about something specific that you observe, then do it objectively. You have knowledge from clients that fill in the blanks for you, but that doesn't mean that you ever have to let on that you know anything from anywhere else.
Your clients are coming to you and dumping information because as a beauty professional part of what you do is listen. When I have a particularly intense conversation with a client, and they dump stuff that is going on that I don't know what to do with, I tend to choose to empower them to take action in their own lives by asking questions like, "what are you planning to do about that?".
This puts the client into the thinking process of figuring out what to do to move forward.
The bottom line is, your friend may have a son who is a bully. You will have opportunities to address that as her friend without ever having to allow what you have heard to influence your choices of when to get involved and when not to. It's just clients dumping THEIR stuff. You don't have to take it on and make it YOUR stuff.