Not really Harten's job to fix her problem for her. If he kills Rex here, the rest of the Rogue's guild will come after him. For the sake of convenience, let's say Harten can easily solo the guild. So, he goes and destroys the guild by himself. Congrats. The guild was most likely a necessary guild to help keep the monsters at bay and you just fucked up the city's chances of survival. Rizrett isn't even an acquaintance of his. So, there's no good reason for Harten to do more than he did.
As for Rex, if he's gonna go after Harten, then he also should be ready to bear the consequences of the results. Harten can easily kill him and his cronies, and Rex knows that. But since Harten is just a passerby and has shown that he's not really associated with Rizrett, there's no good reason or benefit for Rex to ever pick that fight with him. Whether Rex continues to go after the girl or not, it's still really not Harten's problem.
Harten has done all that a stranger should be expected of. He has given her a chance at survival when she had none. The next course of action is her responsibility, not his. If she's smart, she should quickly find someone to take care of the kid (like the church or something), and gtfo out of town asap or find protection from an organization while she has the chance.
