The star has made it clear he is not welcome at the bash in a Spanish hotel on June 30 and, to add insult to injury, guests will include former X Factor co-judges Louis Walsh and Dannii Minogue.
A source said: “Cheryl felt humiliated by Simon and they are not on speaking terms. Cheryl will forgive him, but she’ll never forget.”
Ex-husband Ashley will also not be at the £9,000 do, although they have had a recent meeting.
A source said: “Cheryl and Ashley met and spent a night chilling as mates. They are texting but that’s as far as it goes.”
(Cheryl and Simon)
Cheryl followed Simon as he planned to launch X Factor USA with her as a judge, but her American dream was shattered after she was ditched from the show.
She was devastated when Cowell, 51, and his backers decided after barely a week that she didn’t fit in.
And Simon’s decision not to break the news to his friend himself was an added blow.
The humiliated Girls Aloud star later rejected the chance to return to the UK version of the show.
Cheryl could now take a year-long break from showbiz, with the Sunday Mirror telling this week how the rollercoaster ride has left her hurt and angry.
She said: “I need a break. I need to rest and I need to reassess what I’m doing with my life.
“I haven’t had a break for nine years and I’m exhausted from it all. For once now I am going to take some time out.”