It is difficult when people have diagnoses where they may go on to live many years but will ultimately not survive their disease, it is very stressful for the person not being able to see the future, but also makes family relationships complex- it might be reasonable to expect a family to drop everything and not move if the person has a few months to live, but with secondary cancer now, people can (luckily) go on living for many years, even with a terminal diagnosis.
So- I think this is a slightly different proposition than this being obviously the last time you will see them.
I also think that the son is just trying to do the best by the family, and having dad have a good job where they want to live, is actually a pretty good decision, even though some will be hurt by it. It's not possible always to make decisions where everyone is happy, and I can't imagine having difficulties getting a job/unemployment here (which I have known several people go through recently in the UK) is going to enhance general family life for him and his family.
The remarks about the children crying when mum goes to work are just a red herring, but make me think that the relationship not being close is a two way thing. OP, I work full time and I am a really good mum. 80% of mums work in the UK in some capacity, part time or otherwise. If you are quite so obviously judgemental about them both working, which is usually required these days to pay mortgages and rent (lucky you if this wasn't the case) then it may have undermined their relationship with you. That's a huge shame whether they are in the country or not.