Thanks for all the advise!
To be a little more specific, the Rogue actively seeks out wealthy NPCs, uses diplomacy to convince them to bring her home with them, and proceeds to rob them...using the money she finds to buy herself a shiny new crossbow.
As far as the laws go, i don't think they are relevant because I'm wondering about the morality of the PC, not their lawful/chaotic nature.
I like the idea of adding bluff checks to the diplomacy to make the task more difficult for her. Also, "Lord Pendragon" had a good idea about gradually changing her alignment until she starts being affected by spells/effects that harm evil.