If a person tells her therapist that she knows her husband murdered someone, is that ground for the therapist to report it? How far does patient/dr confidentiality go?
Now that we've gotten through the issue of therapist/patient confidentiality, there's one other issue to cover in your question: the marital communications privilege. It's possible that even if somehow, bizarrely, the patient waived her therapist-patient confidentiality and thus freed her therapist to testify that she said this (or to give the court notes of therapy sessions where this is written down), the husband would still be able to invoke marital communications privilege to keep it from coming into the case. I'm just mentioning this to clarify how incredibly remote the chances of using this statement in court are; with the double barriers of therapist/patient AND marital communications privilege, there is about as close to "no chance" as there can possibly be.
Here's how the marital communications privilege works. If Husband
tells Wife that he killed someone, then Husband has an expectation of confidentiality that Wife can't violate; this is different than if she finds out some other way, such as by seeing the husband dispose of evidence. If the way she found out is by him telling her the secret, then she can't go on the stand and testify "he told me he killed so-and-so." Or that is, she can't unless he waives the privilege, which isn't likely, so even if she wants to testify against her husband, she can't testify about the secrets he told her. This is different from the 5th Amendment spousal privilege of not testifying against each other, which may be more familiar from crime shows on TV... here's the difference:
Marital communications privilege:
* "Owned" by the spouse who told the other spouse something secret (thus, if Husband told Wife something, Wife cannot reveal it on the stand unless Husband lets her). This privilege covers the right of spouses to tell each other secrets and have those secrets be kept. If Spouse A exercises this privilege, he or she can prevent Spouse B from sharing the secret with the court.
* Only covers
communications made in private between the spouses
during the marriage. If he told her the secret the day before the wedding or the day after divorce, she can get on the stand and testify. If he told her any time DURING the marriage, she can't. If he mentioned it a couple of times, once during the marriage and once before, she could testify about the time before the marriage; she might possibly also be allowed to testify about the time he said it during the marriage, if the court decided that he'd waived his right to keep it a secret by telling her before they were married.
* Does not cover anything but communications, and only communications between the spouses. If Wife knows husband killed someone because she saw it happen, or because Husband's friend told her about it, she can testify about it if she wants; those aren't communications between the spouses, so this privilege doesn't apply.
Spousal testimony privilege:
* "Owned" by the spouse who could testify against the other spouse. This privilege covers the right of spouses to choose not to testify against each other. If Spouse A exercises this privilege, he or she prevents the government from forcing him or her to testify against his or her spouse. In other words, it's up to Spouse A whether to testify or not: she can use this privilege to legally refuse a subpoena to testify... or she can waive this privilege and get up there and testify; it's up to her. Unlike the marital communications privilege, Spouse B can't stop her from testifying; it's totally up to Spouse A.
* Covers THINGS OTHER THAN marital communications (which is logical, since the marital communications privilege covers those). Example: if she
saw her husband commit a crime or dispose of evidence, or someone other than her husband told her about it. When she got information by seeing it happen, or in any way other than by having her husband tell her the secret, then it's up to her whether or not to testify against him: she can either exercise this privilege to refuse to testify, or waive the privilege and testify against him. He can't stop her from testifying if she wants to, and the government can't force her to testify against him if she doesn't want to.
* Covers things that happened during or even before the marriage, but only works during the marriage. If Wife saw Husband commit a crime while they were married (or beforehand), but then they divorce, and then he's brought up for trial for that crime, she can't refuse to testify against him--she has to obey any subpoena--because they're no longer married, so she doesn't have the right to invoke this privilege anymore. If they remarry the day before trial, bingo, she can invoke the privilege again.
Note that like any privilege, the exact details of these privileges depend on state law. I'm just giving the general overview here.