OP didn't specify the business, or how many pieces of candy (and business cards) were handed out so there is no way to know the cost and potential return. My FB friends report zero to 500 kids coming to their door last night, I had 17.
I guess we could speculate. Office Depot charges $9.99 for 100 business cards. So if OP's friend had a 100 kids come to the door, she spent probably $15 on candy and another $10 on business cards.
Let's say OP's friend is a real estate agent and gets one new client because of the cards, and that friend buys a house at the average sales price of $313,200. The commission on the sale would be $18.792. Assuming the buyer has their own agent, the net commission to the sellers agent would be $9,396. Now, I realize most agents work for a company and only get about 10% of the commission, so OP's friend might only wee $939.60...but that isn't a bad return on a $10 investment.
Or if the OP's friend ran a quick lube store and got one new customer who spent $38 for an oil change. Granted the profit margin on that oil change probably only nets the OWNER $15 but that is still a new customer that you made money on, and who may become a regular customer.
The classic marketing case is of course Coke or Pepsi. The soda, bottle and transportation on a 2 liter bottle works out to about 10 cents. Everything else is marketing and profit. That's why store brand soda can be profitable at 69 cents, and Coke and Pepsi cost $1.49.