No, I have no words of wisdom because everyone seems to have their own unique experience with CKD and gfr and various other initials. It is certainly possible that his gfr could stabilize for a while; anything is possible, and the weirdest things can happen. I know that doesn't fill you with confidence, but I am hoping that by the time you read this, the neph will have some plan to stop the deterioration long enough so that your husband's fistula can mature. Be sure to let us know.
It is very worrying, and I am so sorry that you have this stress in your life.