Yeah, that look on her face? That's a big part of why he hasn't let Ben have the lightsaber back for good. There are others, but Rey's mistrust is worth considering. An hour or three here and there, he thinks, have been good for his son. Practicing with his saber gives him some outlet for his restless energy. She ought to try it.

"Great."

He doesn't sound enthused, but they'll muddle through.