I think it involves sex. She got drunk & ended up shagging someone- she may not have instigated it or really wanted it but feels at fault for it #metoo
She seems to have recovered comparatively quickly. I thought that she’d experienced some kind of sexual assault, and she seemed traumatised afterwards. But it all seems to be behind her now.
I hope that whatever this turns out to be, her marriage survives. At the time of their wedding, I remember Brian calling it something like a “starter marriage” or “trail marriage”, expecting it not to last, but I think this would be a shame. Regardless of snobbery, Chris is a good guy.