Actually I believe a wife can testify against her husband.
She cannot be forced to do so, but can voluntarily testify
(lawyers at FR please verify).
I wonder if this ‘testimony’ would even be admissible in court, because she wouldn’t be subject to any cross examination by the defense.
I think that the privilege belongs to the husband in this case but it only goes to what he told her. What she observed and/or learned or heard from others is ok.