I don’t just “blame” the mother.
The adoptive son is much more culpable, as he is betraying his real parents who raised him.
His “real parents”(the adopters) are betrayed only if they find themselves betrayed by their adopted son. If they are not betrayed and he continues to uphold them with honor and respect, seeing to their needs and putting them first then what is that to you? Let the Lord most High judge the situation and let it go.
I generally respect your clear eyed down the middle no holds barred views but you are showing an sharp unreasoning edge with this subject. Can’t help to think that this subject strikes to your quick very deeply.