Purplicious, it sounds like she might be experiencing some form of depression, so that might have to be addressed as well. When someone is at such a low point, that they feel such hefkeirus, “i dont believe that there is a G-d running this world” it is more than “I have a problem with Religion”. It sounds more like she feels hopeless. There is something to this story, either something that has happened to her, or too many things that are bothering her that she can’t deal with or cope with. So maybe you would be better off finding her a good Frum therapist who she can unburden and vent to, to first get things into perspective. She might even need some medication for a while to get over the hump.
One needs to recognize the difference between hopelessness and depression and a person who is angry with Hashem and Yiddishkiet. It is NOT the same thing. It is so totally overwhelming to feel hopeless and feel like Hashem has deserted us or given up on us. And that is so totally different that a person choosing to turn their back on Hashem.