I want to say this -- the mom made an effort to get MS evaluated, once possibly twice, iirc. That's dialed in. Knew MS was perhaps struggling but didn't know why. And how could she? Single Mother's (unwritten) Handbook -- never leave your daughter alone with your boyfriend/SO. She may have believed she had done that. She was home, he only took her to/from school, trust grown over time, unaware of what was already happening.
She has to feel completely gutted, for how he used her, gained her trust.
In that interview, I think she was actively, genuinely worried for MS, for what happened after the church.
I imagine the world stopped spinning when LE notified her, first that MS's remains had been found and second, that they had some cctv videos to show her.
Perhaps his vehicle driving past the church without stopping.
Him, throwing away her backpack.
Stomach drop. Her life will never be the same.
JMO