Buddy provides Defense Down, while Evelyn provides Nature Defense Down. These are two completely independent debuffs. As such, the question is actually about the total damage done by these two heroes in each order:
- Buddy, then Evelyn: 3134 single-target damage
- Evelyn, then Buddy: 3203 single-target damage
The reason for this is that although Evelyn hits harder and takes great advantage of the defense reduction provided by Buddy, Buddy only reduces Defense by 34%, while Evelyn reduces the Elemental Defense by 54%. That extra 20% is what pushes things over the edge.
(Attack * Special Dmg % * Dmg Increase from Defense Debuff)
Buddy Unbuffed Damage: 531 * 2.05 = 1088.55
Evelyn Buffed Damage: 727 * 2.1 * 1.34 = 2045.78
Total for Buddy-then-Evelyn: 3134.33
Evelyn Unbuffed Dmg: 727 * 2.1 = 1526.7
Buddy Buffed Dmg: 531 * 2.05 * 1.54 = 1676.37
Total for Evelyn-then-Buddy: 3203.07
…these figures are modified, of course, by the game systems, but still should show a relevant power ratio between the two heroes in question.