Usually, this is just said by a waiter, by someone who serves your meal. A host does not usually say this to a guest.
The usual response is either 'thank you' or no response except a tiny smile. Some people see this as a friendly comment by a waiter, while others see it as a little too informal.
What to say at business lunch/breakfast. So the only way is to say A: Bon appetit. B: Bon appetit. ??? It's perfectly adequate. It's not 100% necesasary that you say anything at all. You can just make polite and friendly conversation.