Spousal Privilege, the legal doctrine that prohibited a spouse from testifying against their partner, dates back to the 19th Century. This “privilege”, which existed at common law, created the spousal incompetency rule, which codified the privilege. Spouses could not testify against their partner, even if they wanted to.
This, however, changed in Canada in 2015 with the introduction of the Victims Bill of Rights Act which amended the Canada Evidence Act to make spouses both competent and compellable to testify against their partner. Compellable means that the Crown prosecutor can subpoena you and call you as a witness – this was previously barred by the spousal incompetency rule. Competency refers to one’s legal capacity to give evidence in Court.
Despite these changes to the Canada Evidence Act, the spousal privilege has not completely fallen away. While the Crown can subpoena you to testify, they cannot force you to testify about communications that occurred during the marriage. This marital privilege is protected by the Canada Evidence Act. You are legally allowed to refuse to answer those questions as they are covered by the privilege. That privilege can be waived, but only by the witness. Meaning that the spouse who is called as a witness has the choice to answer the questions or not. The spouse who is on trial cannot object to the witness testifying as the privilege lies with the witness.
This privilege is limited to conversations that occurred during the marriage. They do not cover conversations before or after a marriage. Further, the spouses must still be married at the time of the trial. The spousal privilege was created in order to support matrimonial harmony, and it no longer needs protection after the dissolution of the marriage. Further, this privilege only applies to legally married couples, it does not apply to common law marriage, as recently upheld by the Court of Appeal in R v Nguyen, 2015 ONCA 278.
The short answer is that your spouse can choose to testify about conversations that occurred during the marriage, and that decision rests entirely with them.
Link for the decision : https://www.canlii.org/en/on/onca/doc/2015/2015onca278/2015onca278.pdf