It’s hard to let them go, but they have to live their own life, and you hopefully did the best you could to give them the tools they will need to succeed. Look at it this way, making such a move early in life lets them sample what else is out there, and no reason they can’t either come back or move on to something else entirely, now armed with two different culture experiences. So much better than being trapped in one place or maybe worse, settling for that, and not see other places at all.
I guess the real question is why is he making the choice. Current conditions in the US? Her? A bit of both? Or maybe he’s gotten a sample of AU and likes what he saw. As long as it’s a good reason(s), and they all are, then why not?
That’s sounds like a done deal then, and he knows what he wants. Honestly, as a parent that’s all you can ask for.