What context do you need? This boy is the first boy, that boy is the last boy. They could be in a queue for something, they could be getting one on one training at the gym and the trainer wants to go home, they could be kids after a birthday party being picked up by parents, it could be a reference to a bag of jelly babies (don't worry if you don't get the reference, you probably wouldn't be asking the question if you did).
The context doesn't matter because there isn't any context in which the translation would be different.
It's not the translation I'm worried about, I can translate the weirdest ❤❤❤❤, I promise. I just couldn't imagine a situation where a string of words such as this would be uttered. (Even in the situations you mentioned, I can't see myself saying this exact thing, but each to their own, i suppose)
Your friend is wrong. "Are you the last boy?" is a copula, liking the pronoun "you" and the noun "boy", and therefore the dependent form of the verb bí would not be used.
An bhfuil an buachaill deireanach agat? - "Do you have the last boy?"
An bhfuil an buachaill deireanach leat? - "Is the last boy with you?"
An tusa an buachaill deireanach? - "Are you the last boy?"