I don’t think it’s so much that “she” was dishonest; rather, a fundamental problem in many relationships are the vague terms of engagement. I could argue that “he” has a duty (in these days of friends with benefits, and no strings attached), to clearly specify his expectation of a monogamous relationship. It’s not as if this couple were actively talking about getting engaged or being married. One’s expectation is not synonymous with another’s obligation, unless that’s what’s been explicitly agreed.