Dirty Rotten Scoundrels (1988)

Answer: She pretends to be a clueless, naive "mark" for them to con. They initially think she is a rich, spoiled heiress (the kind of mark Lawrence usually exploits), and make a bet as to which of them can extort $50,000 from her first. Freddy poses as a veteran in need of $50,000 treatment; Lawrence poses as the psychologist who charges a fee of $50,000 to treat him (Freddy). They later "discover" that Janet is in fact a poor girl who won a contest to go to Europe. This turns them against one another, as Lawrence wants to cancel the bet, since he does not take advantage of people who cannot afford to lose the money he takes. Janet later tells Lawrence that Freddy stole the $50,000 she raised for his treatment, at which point Lawrence, disgusted by Freddy's behaviour and rich enough to make it right, gives her $50,000 of his own to cover her loss. After Lawrence sends Janet away, Freddy arrives and it is revealed that she never gave him $50,000, and trapped him until she could tell her story to Lawrence. Only then do they realise that she is, in fact, The Jackal (another con artist whose name has been dropped throughout the story, and who Lawrence initially believes is Freddy's alias), and she conned them both for $50,000.