My 62yo mom diagnosed with CHF and living with pacemaker, is having atrial fibrillations, esp at night, for which she takes 25mg of atenol. Is it safe?

No. She needs to follow up with her cardiologist asap. While the atenolol may decrease her heart rate, she will need evaluation and possible identification of etiology/possible anticoagulant treatment if she really has a.Fib.
Yes, but... Atenolol will help keep down the rate of atrial fibrillation, but if your mom has what we call systolic heart failure (usually with a low "ef" measured on an imaging test) she'll be expected to see better survival and health benefits taking either toprol (metoprolol) xl or Coreg (carvedilol) instead of atenolol. Coreg is as cheap as atenolol if bought at $4 pharmacies. If the ef is not low, it doesn't matter.