Rendell apologized to Arabella for not being able to protect her. He said he should have been able to sense something was wrong. He was standing close by when it happened but he did not notice anything amiss.
But even Arabella who could hear thoughts was not able to notice that something was wrong. She figured that the one who poisoned her was probably not in the event so she did not hear their thoughts at all.
Though he was caught and was temporarily imprisoned until they find more evidence, the waiter who gave her the wine was probably ignorant that it had poison in it.
"It wasn't your fault. It happened so suddenly," Arabella tried to reassure Rendell but he still kept apologizing. He only left the room when Arabella accepted his apology. He also promised to protect her as best he can in the future.
Arabella did not doubt his words at all. In her past life, Rendell had saved her from several poisoning attempts in Riva. No one succeeded because he was her guard.
