Would you explain the part I underlined below?

She's some dethroned N.Y. debutante and her friends are all in on the deal, too. It's a good scam, as the more confidence women you have the more plausible the scheme seems.

The idiom is "to be in on the deal." They're all parties to the scam.

(That is, they're aware of what's going on, and they're participating in it - on the bad side.)
Thank you. Idioms are killing me.Emotion: boxing