“What is the safest antidepressant for the elderly?”

Any of the newer meds at a low dose, if no response adjust the dose depending on the tolerability, but make sure there are no other reasons causing her to be depressed. Elderly people have multiple factors contributing to their depression, some time you may have to fix the factor causing depression rather treating the depression with an antidepressants.

see this link...
https://bcmj.org/articles/geriatric-depression-use-antidepressants-elderly