I wondered that also and I've been researching that over the last hour or so. In California, it appears mental health professionals are NOT mandated reporters. The following is from CAMFT, California Association of Marriage and Family Therapists
snipped:
To summarize, LMFTs [Licensed Marriage and Family Therapists] are not health practitioners defined in the law to report a client who is a victim of domestic violence. However, other reporting duties may be triggered by the effects of domestic violence if the victim is part of a protected class, such as minors, elders or dependent adults.
ETA: Now IF her therapist wasn't a LMFT, but was instead a psychologist, the rules may be different. I'm still trying to tie this down definitively but having to wade through a lot.