The OP detailed an incidence where the mother refuses to comply with court ordered parenting time by NOT returning the child at the stated time. Moreover, HE spoke of the negative impact on the child's demeanor and a possible long term lasting impact.
I certainly hope that no one is suggesting that a mother can do whatever she damn well pleases without considering the child's well-being, court-ordered parenting time, the other parent, etc.
This may not reach the level of what many would consider Parental Kidnapping but, in my humble and seasoned opinion it certainly constitutes Custodial Interference, Comtempt of Court, and the big one CHILD ABUSE.
This parent should be supervised if there is a history of this behavior or an expectation of future irrational behavior.