Is risperidone harmful for someone who has afib and is over 80?

Risk vs benefits. No antipsychotic is fda approved for treatment in elderly. You have to weight your risk and benefits to see if taking the medication will be more beneficial. All antipsychotics have the potential to increase your qt interval which could cause the afib to get worse. I encourage you to talk to the prescriber about this.
Possibly. All antipsychotics have cardiac and stroke risks. The target behavior must be identified and the shortest duration of treatment. These drugs should only be used if behavior would result in more harm to the patient than that of the drug. These drugs are not fda approved for behavior disturbance in dementia patients. Please consult a geiratrician or psychiatrist to discuss risks and possible benefit.