Not every person gets the same side effects, so this blurred vision isn't common. Don't take any med that gives you blurred vision. Call your pharmacist or doc immediately. There are alternative meds she can take.
It doesn't matter if it isn't common if you're the one who got the side effect, right? Doesn't make it go away. If the Zoloft was added very recently, meaning there isn't a chance of withdrawal from stopping it, the easiest course of action is to go back to where she was before the med increases and see if the blurred vision goes away. If it does, her doc needs to try a different approach.
For me, Zoloft was the worst. Other than the nausea, diarrhea, dizziness, weakness, trembling, no appetite for the 3 days I took it, I couldn't sleep. I obviously had a bad reaction to the medication, and since switched to without side effects. Having blurred vision is a negative side effect for sure and a serious one. She needs to get it lowered or get off it completely. There's many meds out there and the right one will turn up eventually for her, but it's not Zoloft.