The glamour model confronted the Blue pop star over his betrayal - after taking advice from former world heavyweight boxing champ Evander Holyfield.
Her initial attempt to talk to Lee was thwarted by him hiding under the covers with Jasmine and removing his microphone, so she returned to the other HMs and got a pep talk.
Sam Faiers told her: "I'm not surprised you're f**ked off, after that."
And Holyfield advised her to keep her cool, saying: "If he wants to talk, talk. Talk and say, 'Look, hey, I understand'. Hear what he says and take your time before you answer.
"That's it. A man gets mad when he's trying to talk to you and you're just trying to talk... He wants you to hear him first.
"You can always make an adjustment. You know you're smart. Can't nobody make you a fool."
And when Casey confessed that she felt like a fool he reassured her.
"Feeling like a fool don't mean you're a fool, just because you feel like one. Don't get caught up in your feeling. Know that you're not a fool and you hear what he say, and know that any time you do something you did it because you wanted to do it not because you had to.
"You're a smart person. You've got too many doggone options. You don't have to do anything you don't want to do... Can't nobody use you."
She then took Ryan, 30, aside for a quiet word - which was picked up by the sound team.
"All day, I feel like I've been chipped away at and I've not said anything to anyone, I've just held it in," she told him.
"But it's just been going and going and going and in the end I was like, 'I'm pissed off now, I can't take any more because it's going to get worse'. That's why I had to come and speak to you, and I had an attitude because I was pi**ed off."
When he protested that he had no intention of upsetting her, she snapped back: "I would never have been all over a guy in front of your face, Lee, either."
He then said: "I told you, I don't want a relationship with her or you. I don't. I feel like I just don't, not in this house. It's just not where I'm at."
Casey then explained exactly why she thought he was being out of order.
"The reason why I'm so upset is because we kiss, we talk, we cuddle in bed, we are the way we were and then the next day it stops and in front of my face, people have said you've flirted more than you would have normally flirted with a girl in front of my face and just completely stopped with me."
After a bit more to-ing and fro-ing, Lee said: "It's a game babe. I'm not fake. I'm just trying to enjoy my time and be a free spirit in here."
To which Casey replied: "And I want you to be. But I wish you hadn't said to me what was said."
They ended their heart-to-heart with an apology from Lee and a hug.