Your Boss Has Told You That He Suspects That His

Your boss has told you that he suspects that his wife (whose desk is next to yours) is cheating on him, and having an affair with another man. Because your desk is next to hers, he has asked you to check her computer for evidence of this when she steps away from her desk, which she frequently must do.  In the past, she has asked you to watch for urgent emails from customers and others while she was gone, and to page her, which you have done. So even if she came back unexpectedly and found you at her computer, it would probably not arouse her suspicion.  Although you are reluctant to do this, he is your boss, and could make things difficult for you. What should you do? ​

A) Do it even though you're reluctant to do so.  You have nothing to lose and everything to gain. If it turns out she is having an affair, he needs to know, and if there's no evidence of it, he will be relieved - and grateful.
B) Don't do it, even though your boss will may not understand. Tell him that if the situation were reversed, that is, if she was asking you to spy on him, you wouldn't do that, either. Tell him that this is a private matter, between him and her. Hope that he understands.