What if SS threatened to self-harm if she told anyone, and she ran away knowing his phone would be searched and hoping he'd actually go through with it while she was away? Abusers have been known to make statements threatening actions like that; maybe "and then who would take care of your family?"
If that's possible, she may be staying away because he did not get a chance to go through with his plan before his arrest (or never could bring himself to do it in the first place) and despite his being in custody, now there's fear and humiliation on her part. I hope she is somewhere safe and if she's with someone else, they're safe, although a little misguided for not returning her now that he's away for a long time. Or...could they be awaiting more arrests? It didn't seem specific that he was the only adult in the CSAM on his phone.