Well, ideally, they should repent of their errors and sins and pride, make a good and full confession, and commit themselves to a period of penance, and reenter full communion with the Church.
In a lot of cases, that’s not going to happen. Pride blinds. I’m of the opinion that people who are resolute in their conviction that they don’t owe the Church anything, that it has no authority over them, that it’s wrong on this and this and this issue (and of course they’re all issues that have to do with sex, and hey, it’s just totally a coincidence that those issues also happen to line up perfectly with the zeitgeist), that they can do whatever they want because God’s their best buddy and He’ll forgive them for anything without even asking, etc. etc. etc., should stop lying to themselves about what they are and what they believe. They aren’t doing themselves any favors by insincerely mouthing the words of the Nicene Creed (“I believe in one holy Catholic and apostolic Church”) or unworthily receiving the precious body and blood of Christ. If you want to live like a Protestant, well, they have churches for that.