I suspected it because of how complete it was. I had a hard time prior assuming it wasn’t the case because of simple human nature. While the vast majority of Radiants are people who want to help, if it mean sacrificing their own lives to save everyone, they would 100% do it.
But in this case, it’s the potential life of a being they basically see as family. I figured they both agreed to it knowing the cost, or like it was mentioned, they didn’t know it would kill the spren given how complete it was.