I don't know what the ingredients are in Innova Evo - but with calcium oxilate crystals the problem is how the kitties metabolise calcium. The "worst" type of calcium for kitty when they're prone to these type of crystals is greens and I remember our vet saying something about the magnesium levels (which help metabolise the calcium) in seafood - so greens and seafood need to be avoided. No more kitty grass. No fresh catnip. No seafood flavors of anything - no tuna, shrimp or whatever as treats.
Our Tuxedo also had surgery to remove calcium oxilate crystals that had become embedded in his bladder wall.
The problem is not hydration - though it can contribute. Dry food is fine - perhaps suggest she consider purchasing drinking fountains to encourage hydration. Also, we feed the cats one meal of wet food a day, though they free feed on dry all day. She can also boil chicken and feed them the chicken-water to encourage hydration. And meat baby foods make great treats for kitties with this tendency. Ours get it instead of regular treats. And we mix a touch of warm water into the baby food before we put it down for them to snack on.
Royal Canin is actually a pretty high quality food, and IMO there's nothing wrong with the vet's suggestion.