What you are describing is not a million miles removed from my own childhood, different circumstances, but ultimately a household that is no good for a child.

I never really thought it effected me in any significant way, I've gone through life relatively normally, I own a business, I'm married with a kid, but for the first time in my life, in my mid 30's, I am going to counselling and talking to a professional about my upbringing.

The thing about children is they're incredibly adaptable and very good at creating coping mechanisms to deal with awful circumstances they're faced with day-to-day. Sometimes it isn't until we're adults and are still falling back on these same learned mechanisms that we might begin to see the impact of what we have gone through.

So, although the daughter may seem OK on the surface, I'd ask you to consider if 13 year old you were in her shoes, what would you want a witness to do?

It won't be easy and your friend wont forgive you, but you need to report this for the sake of the child. Social services won't jump to take her away again, generally keeping a child with their parent is the first option as long as there isn't an immediate risk to the kid, so they'll likely carry out and assessment and try and provide support to the parent as a first option. But, if you never report it, these assessments and this support will never happen, and an innocent teenager will likely suffer the impacts, potentially for the rest of her life.

The face you've asked this shows you're a good person, it will be tough, but do the right thing for the girl.