Reaver would be great on her, except for the fact that you need to drop 3400g on it before it fixes your mana problem. They should've given it a Hexdrinker -> Maw sort of path, because by the time you get it your mana problems are already significantly reduced and you're essentially gimped compared to someone that bought a 3000g item whose value goes entirely to offensive stats. Having something that could actually help within a reasonable amount of time within lane then upgrade into something useful endgame would be a ton better than what we have now. Make it a 1350g item with the same passive and a combine recipe with a Brutalizer for the end game upgrade. Boom, done, problem solved.
Essence Reaver should work on her as well as any champ in the game, but it just doesn't because it's such a gold sink. Says pretty much all you need to know about the item. That and the fact that pretty much anyone else who wants to fix their mana problem can do it for either 700g or 880g.