It is very difficult to prove shoplifting, especially if they don't catch the items on you. Once you're out of the store, you're pretty much home free. The police can not just show up at your house and search for a watermelon. Most of the items are too easily obtained, and people dispose of receipts. Big, high dollar items can be marked or have identification that shows what they are and where they are from, but if your friend was just stealing groceries, chap stick, or anything similar he should be in the clear.

