Wuddup fam:
I did some digging. I think Darius is allowed to heal. Here's why:
So Fiora's Riposte makes her invulnerable
Invulnerability, according to the league of legends wikipedia, reduces all damage taken to 0. Life Steal heals a champion based on damage done. So, Life steal skills like Nasus passive, Lee sin's Iron Will, and Olaf's Vicious strikes, should not result in healing the champion if they attack Fiora during Riposte. Fiddlesticks drain and Hecarim's spirit of dread probably shouldn't heal either since they heals for a percentage based on damage done.
Darius' Decimate heals a percentage of heal based on number of enemies hit, not damaged. So, I'd expect Darius to heal since him healing isn't based off his damage dealt, but whether or not he hit a champion. So, although he did zero damage to you, he should heal. As a side note, I'd also expect Maokai to heal when he triggers his passive.
