Your friend's suggestion is what is known as a "backdoor adoption" - in other words, it's a sneak-around-the-law method.
I understand your concerns, but if he wants a chance at being Dad, the court will usually give him a shot. His rights haven't been terminated, and he has standing to file for just about anything.
I realize this is a delicate one, but I need to gently advise you to not claim the stress of Dad being around caused your miscarriage. I'm not disputing your statement, rather hinting that the court will likely disagree with you if it's mentioned.
If he does follow through, your family will just have to work it out.

