...and you conspire with your best friend’s wife to kill him...
Of course, if there is evidence to the contrary, that’s different.
Finding the body after all these years, let’s the alligators off the hook and points to the “boyfriend/new husband” as the one who committed the crime and blabbed to deflect some of the guilt onto the ex-wife to get a more lenient sentence. The police cannot probably prove she knew about the crime I would think.