I hate to be the purveyor of bad news, but from the looks of the photos, the roots of your Impatiens have rotted and are unlikely to recover. If you diluted the fertilizer according to the instructions, then that is not the cause of the problem. I suspect that the roots rotted because they were not give a chance to dry out properly. Roots require oxygen as much as they require water. That means the soil around the roots has to be allowed to dry out some between waterings. The soil may feel very dry on the surface, but still very moist around the roots where it counts. You have to stick your finger an inch deep into the soil to make that determination. Your plant is not likely to have needed water more than about once per week.
Sorry, but I suggest you buy your daughter a new plant and let her take care of it.
