You might want to purchase ACSM Guidelines for Exercise Testing and Prescription. It has all the information you need.
Too, you don’t need a special certificate to train older clients unless they are diseased. It’s only important to know what happens to the body physiologically when we age.
All the best to you.
I would make sure you have her health history and a medical clearance if warranted. 150 minutes of moderate exercise per week is a good starting point, as was mentioned above. I recommend picking up a copy of ACSM’s Guidelines for Exercise Testing and Prescription. Even if your certification is not with them, it provides some guidelines for you to follow.