You are rightly worried about your wife. One good thing with depression is if treated adequately it has very good rate of recovery and one can learn to prevent it further. I would suggest, get help of a good psychiatrist, along with that, look at her water intake, food intake, exercise mentally and physically both, create happy atmosphere in house, family activity, family outings, watch a comedy - all this would help. Regular intake of antidepressant under supervision of a professional is also advisable.