If you're asking about what professional would be needed to work with this child for behavioral modification, you may suggest that her mom seeks a referral for Applied Behavioral Analysis clinician through her insurance or Medicaid. Some insurance companies have specific funds allocated for treatment of pervasive developmental disorders.
Initially though, she may have her daughter evaluated by a child psychologist because she's disclosed that her behavior was purposeful. There are different triggers for encopresis ex: anxiety, attention seeking behaviors, control issues,sexual abuse,etc.
Mom may be able to use some behavioral modification at home by using reward/consequence options. The level of cognitive/emotional functioning of the child will dictate what tolls she uses at home. It will be helpful to know what is causing the encopresis in oder to implement a helpful technique with her whether at home or elsewhere. Mom will want to avoid shaming or punishing behaviors and power struggles. When children volitionally defecate, some of them see this as their only form of control. If this is attention seeking behavior of the middle child syndrome, then becoming familiar with that may offer some insight as to how to parent the child at home. Mom can also connect with support groups of parents with children with this condition/behavior.