What does fluoride toothpaste do to the dentin on the teeth?

I've read articles about how fluoride toothpaste is essential to maintaining a healthy mouth, and how it is beneficial to the enamel. However, I could not find articles/papers regarding the effect of fluoride on the layer beneath the enamel, also known as the dentin.

Let's assume a guy who has his enamel completely eroded away (tooth decay), and is only left with his dentin. (Side Question: How likely is this?)

If he starts brushing his teeth with fluoride toothpaste, will the toothpaste help in slowing down his tooth decay, and/or have other positive and negative side effects?