Where did you get this idea from? Drinking water isn't going to help you lose much weight, especially not 10-15 lbs in a month.
1-3 lbs a week is a normal rate for people dieting, water alone isn't going to come close to that. It will help keep you satiated but not enough to cut 500 kcal out of your diet a day.
Water retention is definitely a cause of extra weight. We are used to/trained to not drink as much water as our bodies actually need. Fixing that by drinking a lot of water is a quick way to lose some pounds. Of course it doesn't work if you're already properly hydrated and it only works until the problem is remedied; you will hit a plateau early in your diet. And it doesn't work if your consuming more calories than your burning. But if you're already dieting (which the grandparent was), it's absolutely a good way to kick start your diet.
1-3 lbs a week is a normal rate for people dieting, water alone isn't going to come close to that. It will help keep you satiated but not enough to cut 500 kcal out of your diet a day.