What is proof that treatment doctor wants for 23 week baby will prevent brain damage?

None. There is no evidence that treatments guaranteee no damage. However one does the best they can.
Incomplete question. To get the most from this site you need to provide background information on a problem and ask a related question. Just posting a rather emotional question with no background on the setting, condition, proposed treatment does not provide enough information to begin to assess the issue. You are welcome to start over. Each post must be complete as they are never linked.