Except the game uses Chebyshev distance, so as long as they’re within 30 feet in the x, y, and z dimensions, they’re within 30 feet.
Though for area damage spells, it’s much, much more complicated. You don’t just have to find the Euclidean distance from them to the center. You have to calculate how much of their square is within that distance.
Why do plague doctors get such a bad rap? They risked their lives to help people.