I must say that I'm not selfish or hateful, but if I were in her situation, I would pull the brakes on my SO leaving away, unless he really really really needs to.

If he does not insist, I'd conclude that he doesn't actually want that opportunity so much, and it's not worth the relationship divide.

For instance, you may actually not be so sure after all about this 'dream job'.

if you are, then you have to insist very frequently, because the more she loves you the more she will be against you loving away.

What I want to say is, you two need to have a very deep conversation in which you make her very conscious of what is as stake here, she needs to know how much of a big opportunity that is to you, how it will affect both of your lives, how missing it will negatively affect your lives, and how you will make up for that big distance after moving away, etc, only then, after she is fully aware and considerate of everything, that you can finally make an actual judgement about "dealbreaker".

Good luck you two.