The fact is that he is applying law either intentionally or inadvertently to the wrong place. Law is not applicable to nabuwa. Law is applicable to the life of that boy only. So he is quite right. Law-asha means that that boy has died. It was not for him to live. So law does not open the possibility of his living. But it does not hit at his the possibility of his becoming a prophet. That is a different thing altogether.
The full tradition is law-asha lakana siddiqan nabiya. Lakana siddiqan nabiya is not hit by law. That is a permanent statement. Law-asha means had he lived. So translate it exactly in English and then you will understand better and then you can explain to him. Had he lived he would most certainly have become a prophet. This is the translation. So had he lived tells us that he was not to live. That is law. But he would have most certainly become a prophet. That is not affected by that had he lived.
