Agree with dave that depression among the elderly is common, especially after a long illness or major surgery.

My mother was hospitalized for a period mid 2015 and only recently she regained her previous vigor. Subsequently she had mild depression and tried anti depression medication for a short while (doesn't work and makes the condition worse).

If you were considering anti depression medication for her please be careful as they may not suit everyone. My suggestion is to encourage her to go for daily morning/evening exercise/qigong at nearby parks. Exercise release endorphines which promote positive feelings.