Please tell me, could my grandmother's heart functionality be okay with 28 percent?

Yes and No: The ejection fraction (ef) is low, but this is one measure of cardiac function. Patients with depressed efs are prone to increased complications in the future including heart failure and death in general. She will need medicines to prevent these problems, and on a day-to-day basis many patients can live functional lives at 28%. This is a life adjustment, but not a death sentence.