Andrew Silverman -- Lauren Knows I Didn't Know About Simon Cowell

Andrew Silverman is PISSED after reading the TMZ post Saturday, in which close friends of Lauren told us she's upset at allegations she cheated on him ... because she says he knew about her relationship with Simon Cowell from the get go.

A close friend of the couple tells TMZ ... the notion that Andrew knew about the relationship is ridiculous, adding "She knows this isn't true."

Another thing that pissed Andrew off ... that sources told us Lauren was upset at stones being cast at her because Andrew partied with girls all over the world (though she never used the word "cheating.").

Andrew's friend tells us Andrew says it sounds like "a desperate attempt to smear him."

And the plot thickens. Lauren's lawyer fired off an email to us, saying a lot of the comments attributed to his client are inaccurate, though he doesn't specify which. He also says Lauren and Andrew are trying to settle the divorce privately.

As for Andrew ... the friend says "The two of them have been spending time trying to sort it out and keep things as normal as possible for the kid and this doesn't help."