I don't like where Fiora is right now, but it's not because of Grand Challenge. While it may make more sense thematically for her ultimate to end after she dies, that thing is already hard enough to proc fully, anyways. No one has to jump through that many hoops to deal their damage/utility. The thing I don't like about her is the amount of damage she deals from her AD scaling, much of which is true damage. I could care less about the healing once she finally bites the dust. At least she's gone once she perishes, unlike some folks like Trynda, Yorick, and Karthus.
Basically, Fiora's ultimate makes her current damage easier to predict, and that's about it on the challenge. Meanwhile, you compare that to the other duelist,
, and she doesn't take damage from any other source (while you still do), can soak tower damage for the team, and even gets a pure damage boost meanwhile. While Poppy doesn't get the "flair" at the end, I think any more cutting from Fiora's ultimate would leave it being quite dull and useless. While Poppy is seen as unhealthy for the game, and possibly a poor comparison, you have to realize that Fiora's ultimate is really quite bad. If the rest of her kit wasn't so strong, she'd be in a terrible position right now. I'd suggest instead of increasing Lunge's cooldown on hits that don't hit a vital/kill a unit, or even make her duel give a different effect, such as a damage decreasing aura (from demoralization).
And really, while Fiora does die before the effect, and therefore lose the duel, if her target dies later, it's still a dead icon, like if your Achilles/Hector (Battle of Troy) died in battle. There's no reason the ultimate still shouldn't go off, in my eyes.