Essentially, going after Evelyn at this point in the game leads to a bad path we all generally agree shouldn't be trodden. But, if you wait until you're more skilled with magic, and have more clues to work with, you'll eventually get to the point where you can change her mind. But, definitely ignore the walk through on this subject at this point.
If you've already done it, and she's left, there is nothing you can do to correct it from within the game. You have to roll back to a previous save or start over. Or, you can do a search of the forum. I'm sure pre-Evelyn-scarpers saves have been posted more than once.