Andrew Silverman feels "deeply betrayed" by Simon Cowell.
The New York businessman is still in shock at the discovery his wife Lauren Silverman is expecting a child with the 53-year-old media mogul and can't believe he let their flirtation go on right under his nose.
A friend told The Sun newspaper: "Andrew and Lauren had a great time with Simon. They were friends for over six years and often vacationed together.
"Andrew knew she was flirtatious - it's in her nature. To Andrew it wasn't a big deal. But there's a big difference between friends flirting and friends being sexual."
Andrew filed for divorce last month, naming the X Factor boss as co-respondent in the papers and citing adultery as the reason for the failure of their marriage.
However, family and friends of the brunette socialite - who has a seven-year-old son with her estranged husband - have rallied to support her, insisting she was lonely because the property developer was often away for work.
The insider ranted: "It's pretty impressive that ... [she] would have the audacity to try to lay the blame on anyone but herself."
Simon is reportedly excited to become a father for the first time, but is giving his married lover space in order to reach an agreement with Andrew.
He jetted off to Europe over the weekend, where he will spend the month on board the Slipstream yacht, touring St Tropez, in the hopes that a messy court case will have been avoided by the time he returns.
- Bang! Showbiz