When somebody finally says they regret having voted for Brexit, we are confronted by the problem that there is on the face of it so little that we can do that's positive. Even if we have a repentant Brexiter in front of us, "I realise I was wrong", the automatic response is "It's too f****** late now, isn't it???" and to say so shoutily because there is no other relevant emotion to fit the moment.
So we need to construct something positive to move on with and get the Bregretter to engage more positively with whatever might happen next. As far as Brexit itself is concerned, it really is too late. That is a large part of the problem when confronting the future. The Remainer knows the battle is lost; the Bregretter is confronted by knowing that they can do nothing to undo the decision they made in 2016. The upshot is collective helplessness - a helplessness on which those who brought us Brexit feed.
Getting back into the EU on the terms we had is a chimaera. We're not going to get back in on those terms. If we do want to get back in, we will need to build a majority in favour of rejoining on third country terms, and the majority will need to be big enough and stable enough for the EU to take us seriously. That is going to take a very long time.
But Brexit was not the final goal for the Brexiters; it was always only a stage in the game for them. They are still here, they have nowhere near finished, and they are still prepared to lie, cheat and steal to get what they want.
Our Bregretter, usually, has to start by admitting to having been conned. That in itself is quite a hard thing to do, and especially hard if there is no apparent upside to the admission.
So, perhaps, stage one of the conversation is to say gently, "You were conned weren't you." If they're still a bit reluctant about it, you can say, "It's all right being conned. They've been lying for forty years. They've had half the media on their side, telling their lies for them for all that time. It's not surprising a lot of perfectly intelligent people were taken in."
(As an aside, when somebody complains about the effects of Brexit, it is perfectly legitimate to ask them if they voted for it. The trick is to do it in a gentle and friendly tone.)
They might say, "If only I could vote again". Even if they don't, you can say, you can't get that vote back, but you can be better prepared for next time.
"Because you have to realise there is going to be a next time. The next thing they're going to do is soften you up for trade deals that weaken our workers' protections, or maybe our environmental protections. They'll be working on softening you up to back selling off the NHS. And they'll do it exactly the same way - they'll lie to you, they'll plant stories in the papers, they'll tell the same lies over and over again, and they'll do it for years if they have to. They lied about the EU for forty years to get their way.
"So what are you going to do to stop yourself from falling for it again?"
You might debate around that for a while. (At this point a point of beer probably comes in handy.)
And at the right moment, you say to them. "It's going to take an effort. You can't just say to yourself, 'I won't let it happen again'. You've got to work at being prepared. You've got to start noticing how the right wing press works. Better still, stop reading the Mail / Express / Telegraph - they lie to you all the time. If you're not prepared to give them up, then what you're saying is you don't mind being lied to, and you're setting yourself up for being conned again next time. So you need to be prepared to do some hard work - and I can help you with that.
And then the conversation continues....
The TL;DR version of this is, every Bregretter can be a project. But the aim of the project is not first and foremost to get us back into the EU. The aim of the project is first to turn us back into a democracy.
What about those who voted for Brexit and haven't changed their minds? Debating with them (not "arguing" with them, but "debating" with them) has both purpose and benefits too. Firstly, people do change their minds, but they don't change their minds over something like this as a result of one conversation. It happens most often over a period of years as a result of many, many conversations and experiences. Yours might be one in the chain that leads to a change of heart. You will never know, but, if you choose to do it, it's still worth doing. Secondly, when you debate with someone, particularly on social media, you are not just talking to them, you are talking to everyone who reads the conversation. Even if the person you are talking to is apparently a brick wall, others may not be. The first rule about talking to convinced Brexiters, though, is that is should never be compulsory, whatever the putative benefits. Do it if you choose to, but never feel that you have to.