It's possible that that form of treatment may not have worked for her. Some diuretic agents will strip a lot of water and sodium and potassium along with it and a person will get dehydrated.
Make sure your MIL took the pills in the prescribed manner. If she did, then you need to talk to the doc about the use of diuretic agents in general for you mom. If he gave her pills because of her blood pressure
because the 'return' system of veins and lymphatic tissue gets a little lax as people age. There's no pump down there and the only pump we do have is the muscles in the legs - they 'milk' the fluids back up the body returning blood and lymph back to the heart. The muscular diaphragm also helps - due to it's muscle action on the inferior vena cava. So walking and deep breathing helps all the way around.