You posted your question quite a while ago now so maybe this answer is too late in coming! It might not be that helpful either because I don’t know of any children’s book specifically about adoption but I thought maybe your friend could read the Bible to her little girl which talks about God graciously adopting people into his family. She could even read through Romans and then make her own book to explain it (few pieces of paper, a good black pen and some way of colouring it in a bit and the little girl will love it, I’m sure!)

Just a thought. Sorry if it’s not very helpful!

Mrs A