I visualize this: husband is sleeping deeply, wife picks up the phone and taps the home button while he's sleeping and unlocks the phone. Release the kraken.
If he doesn't have anything to hide from his wife then he has nothing to worry about.
How about a series of emails joking & griping about your wife's habits to your best friend? Completely harmless but not exactly the kind of thing that you'd want them reading.
Our digital selves trail back in time farther and farther these days; would you want your wife reading emails between you and an ex from before you met her?