I have done this for a friend and the only thing is that after his weight went to a desired 10 kgs, he could not control it and now he's got a big belly.
The first question I had when I was about to fatten him was, is it even possible. So The first thing was that I brought him to the gym. He was skin and bones at the time when I first brought him to the gym. The first few months I saw no result. He was still skinny.
I advised him to take some protein. Mass gainers. And he drank and drank and slowly his size grew and also I pushed up the weights that he would carry. I have another friend who has gone up in weights due to weight lifting. The next thing can be done is to increase his calorie intake. If a person wants to increase weight, he has to eat food that is more than what is required by his body. The opposite is true for those who want to lose weight and that is to eat less than what they should be eating in terms of calories. Some of the food that can help gain lots of weight, includes milk , eggs and all those protein. It is not advisable to gain weight using sugary drinks or anything that has sugar. This will result in weight gain but a problem for the body to take care of the amount of insulin in the body.
Well, I think gaining weight is just like losing weight. It has to be done slowly and properly. If there is a weekly gain of 1kg, that would be good enough. But it is not advisable to gain weight in excess of 5 kg a week. That means the body has something wrong that goes on and a doctor needs to be consulted.
Also do check for a doctor. Maybe your friend has got some parasites living inside. heheheeh. Scary isn't it.
I will give you some dietary advice, things to do and not to do to take as much healthy weight as possible. In times of mass gain, eating anything and everything is tempting to take quickly but it should not be done. Taking the wrong kind of weight makes the body misshapen very quickly and especially when talking about sugar, alcohol, cigarette and fast food. Always think about making your weight as gentle as possible because your body is also made up of clogged arteries and systems that can rust in some way. Even if you do not do bodybuilding, I recommend in addition to healthy nutrition, a sport activity even simple (walk) at least an hour a day in period of mass gain.
Keep the body flexible, do simple stretching positions in the morning at sunrise and at bedtime by holding the position from 30 seconds to one minute depending on your abilities: sit on the floor and bend your back to catch your toes / stretch yourself by pushing your arms together upwards and legs like this, down / standing look to bend your back to touch the left foot then the right foot ... You can find a lot of stretching online)
Losing weight is all about mindset. People can do anything they put their mind to, the problem is just that people give up to easily and get discouraged. You can be a motivational force for your friend, someone they can count on or you could also be the person to light the fire under their ass. There are several methods that work. Personally if a friend told me they wanted to lose 10kg I would tell them to envision themselves 6 months from now 10kg lighter, and to keep doing this daily until they are able to manifest it through exercise and healthy eating