To increase your weight, the most important thing is that she did not take any medications, but change her diet, change her lifestyle.
In any case, it would be good visit a doctor to understand what is associated with weight deficiency. Perhaps there is some kind of disease.
To increase weight through muscle mass, not fat, you need to reconsider your diet and be sure to go in for sports (otherwise you will only gain fat).
To achieve muscle growth, you need strength training in the gym. It is better to draw up a program with a trainer to take into account the peculiarities of the body structure, health, etc. Remember that in any exercise there should be 3-4 approaches, each of which has only 6-12 repetitions, no more, in this case you will work on muscle growth, not burn them.
you need it often - 5-6 times a day, with the right food, which will provide material for muscle growth. Those. the diet should contain a lot of protein - up to 2 g per 1 kg of your weight per day.
To make it easier to gain weight, I advise you to buy a gainer, because it is not always possible to cook and eat the right food 5-6 times a day. A gainer contains protein for muscles and carbohydrates to provide the body with energy. It is perfect as a pre-workout snack.
A vitamin complex is also useful. Try Opti-Men or Animal Pak - they are made just for sports enthusiasts who need more vitamins than the average person.
Usually, by the beginning of autumn, I begin to recover rapidly. As I understand it, it is my body that makes fat reserves for the cold. How not to get better? Do you really need to go on a diet every time since summer?