You don't have to be 'nasty' to them. The only requirement is that Lena doesn't offer to help when she hears they plan on closing the cafe. Then, she will have three opportunities to steal money. She can do it two times without consequences, but the third time Ed catches her and tells her Molly has been suspecting her.Never did that myself, but if I remember correctly, you have to be nasty to them, so you will have the option to steal from them.
BTW I always steal from them on my bad/slut Lena playthroughs