Katie stunned fans last week by abruptly tweeting that she was divorcing Kieran after discovering he'd embarked on a seven-month affair with her close friend Jane Pountney.
The shock announcement came just three days after Katie, 35, happily confirmed that she is six months pregnant with Kieran's second child.
Writing in her Daily Star column, feisty Apprentice and Celebrity Big Brother star Luisa said she wouldn't wish this scenario on her "worst enemy".
"Frankly, if it were me in Katie's shoes, I'd cut my BFF out of my life and kick himself to the curb," she wrote. "It's the ultimate betrayal of the two people you trust most in life. My bestie and partner mean everything to me and I couldn't [imagine] that happening."
Luisa acknowledged that some people questioned Katie's decision to reveal her divorce and the cheating allegations via Twitter, but said she understood why Katie wanted to have her say.
"Via Twitter, [Katie] got to set her story straight before the gossip army arrived. Remember, she is the victim here and I think she was right to go public before the rumour mill started whirling and false accusations were potentially thrown around.
"If she wants the world to know that Kieran is a cheating a-hole – and cash in on it – then why the hell not?"
(Katie's rep tweeted last week that Katie had not done any interviews about the split.)
While Kieran has yet to publicly address the situation, Katie's former friend Jane – who is married - has admitted that she did have an affair with Kieran, something that she now "deeply regrets".
Former glamour model Katie married ex-stripped Kieran in the Bahamas in January 2013 following a whirlwind romance and Christmas engagement. Jane was one of the handful of guests at the beach ceremony.
Katie and Kieran's first son, Jett, arrived that summer, while baby number two is due in three months time. Katie is also mum to sons Harvey and Junior and daughter Princess from previous relationships.
It was reported this week that although Katie is furious with Kieran she won't ban him from the delivery room when she gives birth to their second child.
In a statement, a heartbroken Katie said: "I appreciate most people will have seen my tweets on the subject of my marriage to Kieran. I make no apologies for them though it will be appreciated that, sending them immediately after Kieran confirmed to me what he had been doing for the last 7 months, those tweets were a sign of my heart breaking.
"That he was doing it with a close friend of nearly 20 years is hard to understand."
A message later posted to Katie's rep's Twitter account thanked fans for their messages of support, adding: "[Katie] is very grateful and is focussing on making sure the situation does not impact her pregnancy."
Luisa isn't the only celebrity who has been having their say on the situation.
Last week, Chantelle Houghton - who has a daughter with Katie's ex-husband Alex Reid - told Reveal she was shocked by the whole thing.
Echoing Luisa's words, she said: "I think that Kieran and Jane are scum of the earth. It's the ultimate betrayal, there's some real nasty people in the world and there's two of them right there. It's disgusting. To be pregnant and that be going on, with your best mate. It's not like she's only known her for a short time, she's known her for 20 years.
"To do that to another human being, there's just no words."
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Katie's ex-boyfriends in pictures:
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