I am not sure where to go here but I would suggest having a talk with your daughter in law and asking her why he thinks this. If you feel sure she is telling the truth I would tell her to give him the most positive reinforcement and also search the web for some information on old myths so he can see it in black and white.
Some times when men are incarcerated they become very insecure. They are lonely, frustrated and want to lash out there frustrations in this case it sounds like he is lashing his wife. I would also point out to his wife that this behaviour of his is not healthy so maybe she can get him some help while he is in there.
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