Definitely definitely say something.

My dad is a builder and he has always spoken to me about the dangers of dodgy work. What if he leaves a wire exposed and a child gets electrocuted? A dodgy railing and a child falls 2 storeys. It's not just money for yourself and others you have to think of. There are so many bad things that could happen if this man is obviously careless in his work.

I would go anonymous like you said, but I think you would feel awful if you found out something terrible happened and you could have said something. If you're friend is a true friend then be there for her through whatever follows, but make sure you do what you know is right