"Is it true he can't die?" Obi-Wan asks in return. He's not overly bothered by the idea that Vanya is practicing her abilities in a safe manner, using them in a mostly controlled environment on someone who truly cannot be wounded in the process of it.

How else is one meant to learn how to deal with what they have? It would be like telling Force sensitive children not to practice on one another, when Obi-Wan is well aware they do. He had participated in it plenty himself, had even once thrown Plo Koon a little too far in the midst of a game.