I agree with Matt's interpretation. Just want to add that 'spontaneous friends' isn't a 'real' English expression, even though we understand what it means in the context.

It sounds like the restaurant is referring to their customers as "friends" to sound more intimate. "Spontaneous" describes something that occurs without planning beforehand. Therefore, "spontaneous friends" are customers who arrived without a prior reservation.

Thanks a lot. All answers are very useful for me.