Lex did not need to expend any energy at all to see through River's entire body and cultivation. He was a mere mortal, with signs that he had previously been in the Qi training realm. Even if Lex had only a pathetic amount of energy, he still had the foundation of a peak Heaven Immortal. The things he could do with even that small amount of energy was beyond River's imagination.
Lex could also devise the perfect cultivation technique suited to River if he so wished. Not only would that allow him to embark on the True Path of cultivation for humans, it would also allow him to cultivate with speed greater than if he was merely following a single path and using a generic cultivation technique.
Still, he chose not to do that.
It wasn't because for cultivation River would need to heal his meridians first. For Lex, healing River was not an issue, though he would have to use up his spiritual energy to make that happen. No, the reason was something else entirely.
