Just have Beniang eat Agni's crystal. It's a crystal of fire, so it should be excellent food for a dragon. She could probably rebuild her dragon heart and make it stronger than ever.
But knowing Frey, he will just leave Beniang as a permanent invalid, never expecting anything more from her, except for menial tasks. That's how he's been treating Isabelle all the way, after all. Dude trusts nobody with anything important and has to handle everything important by himself to be satisfied. Even though he would be already dead without the others.