Did they transfuse Rh Positive blood during his surgery? That's the only reason he would make Anti-D. 13 units is a lot - must have been a massive transfusion protocol where they automatically switched him to Rh Positive Blood.
I do not know. That's a question for experts. But I do know that,
For all have sinned, and come short of the glory of God; Being justified freely by his grace through the redemption that is in Christ Jesus: Whom God hath set forth to be a propitiation through faith in his blood, to declare his righteousness for the remission of sins that are past, through the forbearance of God; (Romans 3:23-25)