Admittedly I am only on page 3 (of 100 posts per page) but I think you are being judged unfairly, OP. I too feel it is very selfish of her, and I am an only child. I am a carer for my mother and would like to do a teaching degree. But, I'd have to move away, and considering my widow mother has Bronchiectasis and has been clinically dead at least two times and can get critically ill sometimes 5 minutes after being ok, she needs me here (though she says she'd cope, she can't drive and needs me to drive her to appointments, so, I don't think so).
It depends on how close one is to one's parents. I could never, ever, ever move away to another city, let alone country. I could never do it. I would feel so selfish, so guilty, and so cruel. So, I can't do my degree. But that's life. It happens. We all have choices. I choose to look after my mother and be there for her. I choose her, and her welfare, over myself. That's not to martyr myself, it's just that I could simply not live with myself. And wouldn't be able to concentrate on my degree, anyway.
In many cultures, children put their parents first. I don't see that as a bad thing. If her partner really loves her, he'll wait and stay with her in the UK. As you said, she doesn't even like his family. She has no job. It's rather irresponsible of her to move over there, with no job, and no way to support herself. Neither of them have jobs lined up. Being Australian myself and knowing what our border control and immigration processes are - heck, you need money to support yourself even on a holiday visa - I would be surprised if customs and board control allowed her to enter the country with no means of supporting herself. Just watching episodes of our real life tv show Border Security (that shows people sneaking in on wrong visas, or trying to smuggle in drugs or food items) shows how they ensure any holiday makers have enough money to support themselves. So, she may not be allowed to enter - or, if she can't stand his family, she may not last here for long.
She may be back sooner than you think.