I would tell Sam that shoplifting is wrong and that just because she didn't get caught doesn't mean she can always get away with it or it doesn't mean anyone else can get away with it. There could be serious consequences to it if you get caught, it could go on your record which could affect your life and career.
"That offer is tempting but I'm good, I'd rather just pay for it, it doesn't cost much, besides I don't have to worry about causing any trouble for myself and others."
You could get caught and they could charge you plus they will add it to your criminal record which will stay with you forever and it could affect your career and ability to get accepted at any job or school.