For the most part, weight gain is simple mathematics, you take in
m ore calories than you burn and the excess is stored in
fat cells. Your mother can do some sort of exercise to burn calories, she just needs to watch her heart rate and work load. Just moving burns calories so a simple walk will do it. If walking is too much, some time moving in a pool is very helpful.
In any case, she needs to cut back on her calories to match what she burns in a day.