Let's make a recap.
Pedro defeats and kills his bastard brother Enrique. Only defeating him doesn't help him. He also MUST fulfill his promises to the Black Prince to keep him at his side. Otherwise, he's going to loose the war and his life, as IOTL, because France is going to help Enrique de Trastamara to go back to fight. And this time, Pedro would loose.
He also need to have killed in the battle or in the aftermath the two brothers of Enrique, or France is going to repeat the trick again.
So, let's suppose that Pedro pays his debts with Edward, who can return to England not broken and without having to heavily tax Aquitaine. He returns in better health and England has a powerful ally in Castille, which is a pain in the arse for France. The Anglo-Castillian fleet simply rule the sea and France is royaly f***.
How long he would go on? Perhaps, after too many wars, he would settle for a while. As long as he has the English on his side, France is not going to attack him. IOTL Enrique only returned with an army to Castille after Edward returned to Aquitaine after Pedro's didn't fulfill his promises. If keeps his word, Edward keeps supporting him and France has a lot of reasons to be quiet. Pedro must be remain calm and using his brain, not his balls. And that was not easy for him...
If France tries to send one of the brothers of Enrique to the fight, things can become nasty, if the English are not capable of sending any help, something that I dobut. So...
If the French grow balls (only if the Black Prince is not around or unable or unwilling to return to Castille), they my try. But they need a pretender. The question is: who?
-Tello de Castilla run away after Najera. He was too umpredictable, perhaps not too trustworthy.
-Sancho Alfonso de Castilla, unmarried and without heir. A nice political toy, one would guess.