When your ex-wife admitted him did she sign the admission papers or a contract? I am not aware of any law in NY that would impose liability on a parent for services rendered to a non-cohabitating adult child outside of a contract -- that is also assuming that the rehab center is in NY. It may be out of state and that state's law may come into play. For the time being, I would analyze this as a contract question. Did you agree or represent to anyone that you pay this? Did you implicitly make such a representation to you ex by agreeing to this course of action? Did you have any involvment at all with the decision to admit your son into rehab? If there is not agreement or representation that you would contribute or pay, I would look to the rehab center's admissions policies and papers to determine how they expect to get paid and whether your ex represented to the center both parents would contribute or assumed that liability herself.