I think that you have to think very carefully about emigration when you are old.
For a start, Brexit has obviously made a difference, as it may be more difficult to make the move and things like healthcare are likely to be more expensive than they used to be.
It's nice that her son is financially willing and able to support them. However, there are several things which might happen to change the situation. He might lose his job. He might get divorced and have a second family to support - these things do happen. He might become ill and unable to work. He might even, God forbid, die. Or your friend might need treatment for serious illness or long-term care. That doesn't come cheap when it's private and it's not always easy to get health insurance when you're old. Your friend's son could end up with very big bills to pay and, although he may be willing, she might feel bad about it.
There's also the possibility that, when people move to be nearer to their kids, the kids might get an opportunity which would involve them moving away. That could mean moving again, staying in a place where you didn't know anyone, or knowing that your family had given up an opportunity for your sake.
I realise that these are all gloomy scenarios which might never happen. If they did, I suppose that it would be possible to pack up and come home. I just think that it is important for anyone to bear in mind, when considering this sort of move, that their family's circumstances might change and so might their own. A couple in their early 70s in good health can probably deal with change fairly easily. That might not be the case further down the line, when they are older and frailer and maybe on their own.
Sorry to be such a pessimist but anyone contemplating such a move has to think about what might happen and how they would cope. Particularly when they are older and find it harder to bounce back.