I get why he’s minimising, but fuck me i can’t get why Lillie is kicking Millie so hard.
My guess would be that she's angry Liam chose Millie over her and has decided to get her revenge. I think she probably believes what she's saying, and has deluded herself into thinking Liam liked her a lot more than he actually did, and that its Millie's fault she wasn't chosen. She feels like she's the woman scorned.
Otherwise I can't figure out why she'd embarrass herself by agreeing to have that meeting with Millie.
The producers could have found a number of other ways to let Millie know what went on. Twitter challenges, headline challenge, even a 'what happened next' like they did in season 1.