It seems like you're stuck on the specifics of the word "dishonesty." You're right, the person in your example is being honest... and he's being emotionally abusive, and he's cheating. Relationships are an agreement and she has not agreed to an open relationship, so he is violating the rules of their agreement. That is called cheating.

Realistically, all relationships are societal constructs. The problem is that society has come to expect monogamy as a sort of "default" rather than making it an open negotiation. So because the unspoken rules of dating in our society state monogamy, having sex with another person is cheating those unspoken rules. One person can not unilaterally change the terms of an agreement however, so it's still cheating to declare new terms and act under those without the other person's agreement.

Ethical non-monogamy (aka polyamory) is about being open and honest when starting a relationship as well as throughout. Under this paradigm you discuss the terms of the relationship before entering into a relationship, and continue to do so during the relationship. It's about agreeing and sticking to the rules of said agreement, and renegotiating them and coming to a new agreement if the rules no longer suit the participants.

Also, your friends are assholes. :)