Well, I don't know all the details of this thing.

But, unless:

1) There was a HUGE, impossiible to miss sign above the hot dog preparation area, and

2) You had previously required the customer to sign a statement noting that there should only be one hot dog per bun, and

3) You specifically had him initial the area next to that part of the statement, then

I strongly doubt that you could prove the customer was doing anything illegal.

The employee, upon noticing the extra hot dog in the bun should simply have rung up the extra charge for the additional product. No crime has been committed until the customer leaves the store without paying for the product.

Of course, if you hadn't properly trained your employee how to handle the situation, you can't hardly fire them for it.

It's all about training your employees to provide excellent customer service.