Grammar
사람이 어디 있어요
who would do such a thing?
Form
V-는 사람이 어디 있다? — rhetorical question: “what kind of person would do that?”.
Function
Rhetorical question with 어디 있다 implying that such a person is hardly conceivable.
Why this form?
누가 그러겠어요? — “who would do that?” also expresses disbelief, without framing a type of person.
How to build it
V-는 사람(이) 어디 있어요? uses a present noun-modifying verb form. Here 어디 does not ask for an actual location.
Forms not to confuse
누가 그러겠어요?
누가 그런 짓을 하겠어요? — Who would do such a thing?
Examples
남이 자는 걸 지켜보는 사람이 어디 있어요??! — What kind of person watches others sleep?!
그런 말을 하는 사람이 어디 있어요? — What kind of person would say that?