I would be devastated if my DCs were to date somebody on the far right, I would feel like I had failed miserably to teach them about respect, equality and acceptance of difference, I would do my utmost to keep open conversations with them and try to figure out what was going on, I wouldn't give up on my DC no matter what they did, but I would find this very, very hard
Of course everybody is "entitled to their opinion" but that does not mean all opinions are equally deserving of respect, if by far right you mean xenophobic, racist, homophobic, islamophobic, against democracy and all the other lovely things found in that ideology then I feel your DD should be ashamed of herself for associating with him
yes, he has a right to free speech (as long as he is not inciting actual violence!) but we also have the right not to associate with him because of his opinions, the fact that your DD choses to be his partner would make me worry that her own political opinions either already are more extreme then you think or that they will shortly become so
are you sure that her objections to migrants genuinely are about legality, or is she one of those people who will say, "I am not racist, but..."? usually a pretty sure sign that the person speaking is indeed racist!
Also, do they actually know what they are talking about? because very often people will bleat on about unregulated migration without realising that actually seeking asylum is an international right recognised in law, and that coming to the UK without papers to seek asylum is not illegal AT ALL!
In fact, do you know who the main group of "illegals" are often in Australia? Brits who overstay their working or holiday visas! But who is in detention? asylum seekers who have done nothing wrong by arriving by boat to seek asylum (and in most cases have done nothing wrong at all!), and have in fact, just exercised their rights! Great example of how to manage migration indeed!