Why not take this statement at face value? Misty was babysitting and told Ron that someone came into his home and took his daughter. If he's blaming Misty, it's for letting his "daughter get stole." Even the sentence construction--very unconscious in speakers under great emotion--is passive voice, indicating that he doesn't KNOW who stole the child. If the sentence was just a statement--"My daughter got stole"--the passive might also indicate RC was avoiding stating his own responsibility, as that is one major use of the passive. But the other use is when the speaker does not know who is responsible. In this case, the passive statement is embedded in a question; Misty is the subject ("you" since RC is speaking directly to her); "let" is the verb, meaning "to allow" and the object is the passive sentence "my daughter get stole." And all of that is positioned in a WHY question indicating that RC doesn't know who "stole" Haleigh and has already accepted that Misty allowed Haleigh to be stolen, at least in the first emotional moments after he was told what happened. He's not asking what happened to his daughter; that in this moment seems obvious to him. His question for Misty is why she let that happen. And that's surely a good question. That response might actually be a strong indication of his first thoughts and his first instinct, before his usual denial and bad judgment got him in still deeper with Misty.
It would be very tough for someone to "fake" that kind of syntactic structure and get the sentence to come out "right" in spite of Ron's non-standard use of "stole". Even that word "stole" is telling, if less than Standard English, with it's implication that she "belonged" to him and someone unknown took her away. In a spontaneous utterance like this, statement analysis says to look at places where the speaker can't get the syntax right, or the syntax doesn't match the message, or the syntax shows a different underlying relationship between agents and objects. Here it is a dead match for what he says: Misty let someone "steal" his daughter by a person unknown to him at that moment.
Thus, you are right: he is not denying her "involvement" in terms of not protecting Haleigh from whoever took her. That's clear as crystal to me.