Second child off to Australia

Why would you blame this young woman for a decision your son has made? He’s 28 and exploring his options. He shouldn’t be held to a statement about moving away that he made when he was younger.

You need to accept this with good grace, make the woman welcome and then wait to see how it pans out. I doubt anyone is going to be moving anywhere soon so who knows, it may fizzle out or otoh, it may be the perfect match for your son and you should be happy that he is happy.

There is nothing in your post OP that says you are happy for him to have met someone he loves, someone he wants to bring to meet you & wants to share his life with. This should tell you all you need to know about your relationship with him I'm afraid.

What has this girl done wrong to warrant your animosity. Your son obviously wants to be with her and would like you to meet her. I take it he is single and it wouldn't mean a marriage breakup? Ok, he once said he wouldn't move away but life changes and he has to do what he feels is best for himself? I can sympathise as both my children moved to Australia.

Why on earth do you feel you could not bring yourself to be pleasant to this girl?
Do you feel she is solely to “blame” for your son’s decision?
Admittedly Amsterdam is a lot nearer than Australia, but did you raise similar objections when he chose to live there?
And I’m afraid I cannot see how your daughter moving to Australia 10 years ago somehow led you to expect a closer relationship with your son - married or not.
Your logic escapes me and I think you and your wife have some hard thinking to do about what your relationship with both adult children involves.
I don’t mean to sound unsympathetic, I am honestly not, but feel you are creating unhappiness where it need not exist.
If you cannot be “pleasant” to your son’s girlfriend, do you really imagine that is going to persuade him to stay?
More likely you will lose him too. Please think this through.