First off, king size candy bars are only like $1-2 at
Walmart. I absolutely do not believe that the candy was $5 per bar, and if it was, I think it was terrible business sense to purchase it when you can buy a regular size Snickers for a dollar or less. She will be out of business in no time if she spends like that.
Second, it's tacky to attach any advertising to the candy. 99% of the cards will be thrown out anyway by angry parents or by sugar crazed kids who couldn't care less. If she doesn't want to give out free, no strings attached candy, then just don't participate.
Frankly, I think she is missing a better marketing opportunity...setting up a bounce house in her front yard. Put a sign out encouraging kids to jump around for free that night. Granted, it may be difficult for kids in costume to do this. But whether or not they jump, everyone will see it. I think it would be fine to have a company sign next to it advertising rentals. This way, the candy doesn't feel like payment for reading the business card. And you are letting people sample the product for free. But really, take the cards off the candy...you will end up turning away more potential customers than you will draw new ones in.