Theft?

I don't know what to think. A fellow cashier came through my line the other day, and bought a hitch with a ball bearing. I accidently only rang up the hitch, since the other part was attached and I thought they came as a set. She tells me nearly a week later that I forgot to ring up the other part, which cost nearly $40.

She also "warned" me that if either of us speaks up, it is regarded as theft, and we are subject to disciplinary action, possible firing, and legal measures. She said that theft is a misdemeanor that can mean up to ninety days in jail and $3,000 in fines, plus a mark on our permanent records.) I, however, think this is an elaborate ruse to get out of paying the extra $40, and a way to make sure I stay quiet. She is pretty strapped for money.

Also, she stands to lose her job for using her discount card on this item. She told me later (while warning me about legal repercussions) that she actually bought this for a friend. Her tone and secrecy makes me think that her friend reimbursed her. This is just one of the ways you can get fired at Walmart.

Given that it was an honest mistake on my part, and perhaps a not so honest mistake on hers, do you think I should speak up? I doubt that nearly anything she said is true, and don't want to rob my employers of the $40. However, if she is subject to disciplinary action for this, but keeps her job, that would make work miserable for me. Oh, the lovely conundrums.