I agree with m1k3, but, I might ask him though, if he was getting tired of the obsessive detective work. I did detective work for far too long...it is very draining. Ask him if it drains his mental energy. Then suggest he simply let her do whatever she is doing and not do detective work on it, since she is going to do it anyway.
I'm just thinking of ways to help him.
I might say--when it becomes too draining dad, you might want to consider telling her to leave, or trying an alanon meeting for yourself.
And you could anonymously leave "codependent no more" on his coffee table.