Even though it's extremely common to do so, 5th edition assumes you are not playing on a grid, by default. Using a grid is proposed on PHB 192 as a variant rule. In that rule, it says that moving to any adjacent square (including diagonals) counts as 5 feet, period:
Entering a Square. To enter a square, you must have at least 1 square of movement left, even if the square is diagonally adjacent to the square you're in. (The rule for diagonal movement sacrifices realism for the sake of smooth play. The Dungeon Master's Guide provides guidance on using a more realistic approach.)
This means, regardless of actual math, that we can measure your distance traveled as two straight lines, distance forward, and to the side. Whichever of this is longest will be the equivalent of how far you've moved.
You would measure other distances the same way, like how far away a flying creature is, or how far we need to shoot downward at a target.
A simple example of this is if I move forward 10 feet and to the side 5 ft (2 squares by 1 square), I've moved 10 feet.
So for your example, if he is 110 feet up, but only 25 feet away horizontally, the distance would be measured as 110 feet.
This is used for simplicity's sake but is not mathematically accurate. Aware of this, the designers proposed an alternate method in DMG 252 (as mentioned above):
Optional Rule: Diagonals
The Player's Handbook presents a simple method for counting movement and measuring range on a grid: count every square as 5 feet, even if you’re moving diagonally. Though this is fast in play, it breaks the laws of geometry and is inaccurate over long distances. This optional rule provides more realism, but it requires more effort during combat.
When measuring range or moving diagonally on a grid, the first diagonal square counts as 5 feet, but the second diagonal square counts as 10 feet. This pattern of 5 feet and then 10 feet continues whenever you're counting diagonally, even if you move horizontally or vertically between different bits of diagonal movement. For example, a character might move one square diagonally (5 feet), then three squares straight (15 feet), and then another square diagonally (10 feet) for a total movement of 30 feet.
If you aren't using the Grid variant rule, you would just measure the distance from point A to B.
You can also, of course, actually do the math as NautArch suggested in his answer. It comes down to what you and your players can agree on. But either way, whatever rules you set for measuring diagonals should be used for all cases of measuring diagonals. As long as you're consistent within your own game, that's what matters.