I do not think it warrants quite a dramatic response as immediate walking away... Yes, the liklihood is that he does not respect her and all the other things that Miner says are true and so the relationship will fail. However, a sensible and mature response is to have the conversation and give him the chance to explain himself. I have to say that it does not look good. If he'd made it clear he wanted to be poly or even if they'd never discussed it and she just assumed then there may be a way out that leaves them still in a relationship but there is the case that he told her he wasn't going to do this then did. What he should have done was informed her that he wanted to change the nature of the relationship and that the initial conditions were changing.