One of my friends is very slim and is not healthy. He wants to gain health. How can he get a little fatty or fatter? Which type of exercise and which type of food, he should consume to be able to gain a little more fat?

I wouldn't worry about gaining weight per se; weight alone is not an indicator of health. Simply exercise and eat healthily and let your body determine your correct weight. That being said, if you eat more calories than you burn it may be possible to gain weight. And eat more potatoes and meat (although too much meat is bad for you) as opposed to veggies and fruit. Also, I would recommend more weight lifting as opposed to cardio to gain weight.

Proper food, no prepared foods, lots of fresh veggies or fruits

Dates with glass of milk in the morning, might work for him.