My ex has discussed online having hallucinations and schizophrenia. Can she get custody? Her hallucinations involve the forces of good and evil battling. She has previously thought he was part of a foreign government and was being followed and sacrificed.
I would first advise you to preserve your proof. Keep screenshots of the Facebook posts if they are still available. If litigation is pending or commences, you can ask that the court order her to participate in a mental health assessment. If she has ever been treated you can ask that the Court order her to sign a HIPAA release so that you can obtain those records.
If you have court orders in place, these mental health issues would certainly provide a strong basis to seek modifications restricting her parenting time and her right to participate in major decisions.
If you have no court orders in place, AND Mother is seeking access, then you could and should seek temporary and long-term orders restricting her parenting time until he is able to get things under control, if ever.