How to give a free product to customer that refers a friend?

In order to give a customer a free product when they refer a friend, you need to configure a coupon amount to be equal to that product price, and also add the product ID so that it can be applied only to that product. Check the image below to see an example of such configuration:

Under plugin settings -> Coupon Settings tab, select Product Discount as the coupon type. Coupon Amount should be equal to the product price - the product that you want to give for free. And lastly, under products, you need to add that product ID. You can get the product ID by hovering the product name on the Products screen, as shown in the image below: 

That's it. Now, go to the Email tab, and under "Email message", write some text so the customer knows that the code they received will give them a free product.