My DB has just broken up with his girlfriend of 12 years after he discovered she'd been cheating on him for several months with a close friend. Turns out she's had a couple of one night stands previously as well (which he knew about apparently, but no one else did).
DB had a sort of non-reaction really. He had been questioning the future of the relationship a bit himself, but he doesn't appear to be angry with her at all or devastated either.
They still have to have some contact due to work, and she's also still technically living in the flat they shared. She's been staying with a friend, though has been back to the flat as well.
She will be going away for a month or two in January. He recently suggested that she would "see how she feels when she gets back" and it sounds like he'd take her back if she wants.
How can I help him see that the future does not contain this cheating witch and he should leave this relationship behind him? He's stuck in a sunken costs fallacy. I think he's been feeling a bit lonely over Christmas and he's been texting her...