If you are on a liquid diet for medical reasons, you can still gain weight if you need to. Don't fill yourself up with no- or low-calorie beverages, like diet sodas. Fruit juices are good. You can make smoothies, or drink commercial weight-gain products. Many weight-gain products are mostly protein, but you have to make sure you get balanced nutrition with enough carbohydrates and protein, plus some good fats. An inexpensive protein source is nonfat dry milk powder, and you can get soy protein powder or drink soy milk. Frequent, small liquid meals will be better than large, filling ones. When you are choosing a product, take the one with the most calories, as long as the extra calories don't come mostly from fat. Do some weight training if you can so you will gain more muscle and less body fat.
Try adding some extra calorie-dense ingredients to whatever she is eating. Fruit juice can add taste and calories to bland food without being filling. Some protein powder may help. Nuts and seeds are good, and so is avocado. A little olive or canola oil can add calories, as long as she likes the taste. Look into baby food. Some adults find it quite tasty. See if your mother would eat more overall if she had several small meals throughout the day.
Water itself does not cause weight gain, though you should continue to drink the 8 or more glasses daily that is generally recommended for good health. If you drink a lot of water--more than you need--you will gain weight temporarily, but you will lose it as soon as you sweat or urinate away the excess.
If you can't gain by eating, look very carefully at what and when you eat. You may need to eat more or less frequently, or choose foods that are calorie dense. Liquids don't fill you up as much as solid foods, so juices and smoothies may help. A registered dietician can help you plan a weight gain diet. For exercise, you should be doing only muscle building weight training. Try working your whole body two or three days a week. Concentrate on big muscle exercises and sets of eight to twelve reps. Lastly, ask your doctor to check your thyroid. Thyroid problems are not common with young men, but it's worth checking out.
There is really no drug or hormone for general weight gain. Growth hormone is very expensive and has lots of side effects, so should be reserved for medical needs. The growth hormone you see advertised on the Internet is not real GH. If it was it would be illegal to sell, as it must be prescribed by a doctor, probably a specialist. If you are not having results gaining through eating more and exercising, check with a registered dietician who can see how many calories you're really taking in. Just eating until you're stuffed may not be eating the right things.
If you gain weight, it's going to be either muscle or fat. There will really be some of each, but you want it to be mostly muscle. This doesn't mean you will have bulging muscles. A moderate weight training program will firm and tone, without giving you a lot of size. It takes a lot of work to get big muscles, and isn't really possible for most women and some men.