In a strong monogamous marriage you don't intentionally do things or accept other's behavior that could lead a reasonable person to draw the wrong conclusions. She isn't cheating (yet), but she knows that she is doing something that would hurt her partner if he knew about it. So when she's taking a shower and he sees an inappropriate text from Chris on her phone and goes digging, do you think he would be more concerned with the responses she did make, or the missing text messages from times when he knew they were texting. Having to say "It's not what it looks like" isn't a situation you put yourself in intentionally, because more often than not it is exactly what it looks like, and no matter whether she actually crossed any lines or not, their relationship will be on less solid ground than it would have been otherwise.