During the discussion, he prodded me on where I thought moral law came from in my metaphysics. I talked about morality as though it were some kind of Platonic form, remote from the plane that humans existed on. He wanted to know where the connection was.

I believed that the Moral Law wasn’t just a Platonic truth, abstract and distant. It turns out I actually believed it was some kind of Person, as well as Truth.

Presumably asking her to clarify her belief that Morality = Person, and discussing other options for Morality and why they do not make sense to her as much as the Person one could be a start. Though I'm guessing that it will end up at discussing reductionism, something she is "skeptical of".

Or maybe she just craves more love than she gets, and applying logic is a wrong approach to begin with. Maybe get her to attend one of the more welcoming and less judgmental LW meetups in the area.

EDIT: having looked some more through her blog, she seems (hindsight bias notwithstanding) to have been a closet theists, so this is basically a coming out party for her. You are probably out of luck. Your best (?) bet is that she will run into some hateful zealots who will make her question her beliefs again. Though for her sake I hope that she does not and lives her life as a happy Catholic. Sometimes happiness trumps rationality.