Lancome my advice to your friend is to get some counselling herself, from someone who understands about adoption and to aim to have some family counselling with her ex and her son. She needs to find a way to move on, to allow her son to live in the best place for him. He is almost (less then 4 years away) from becoming an adult, so in less than four years he would be able to decide for himself where he goes.
There may be much more that you are not telling us and more still your friend has not told you.
I have no vitriol for your friend, or anger at how she feels the need to express herself to you, one adult to another. But I sincerely hope she will get counselling before she broaches any of this with her son. He has suffered rejection and does not need more. I am sure in her heart she knows this.
As others have said, she cannot unadopt him. Even if she could this would not, I think, provide the closure she may feel she needs. She needs to find a way to deal with the issues and she needs professional help that should be available free from her local authority if she is the UK (as far as I am aware) although she may need to pay to access it and I feel it is essential for everyone that she does so - much cheaper than boarding school anyway.
Lastly, we all make mistakes. I am mum to a daughter by birth and a son by adoption. I make loads of mistakes with both my kids. I love them both intensely but we are all human. The thing to do is to find help when we mess up, ideally before we mess up too big!
And, Devora, I don't know you very well but I do a little - and feel you are someone who does the best she can, and work amazingly hard, and you were a total inspiration to me (along with Kew and others) before and after we adopted.