from my exsperiance of playing the game. eat some of the fruit saled and something fatening or savery eventually your capacity should grow so you can eat more then one thing fattening and still a little bit of fruit saled. Do not over eat past the bar that gets you sick if its more then half the bar overfed (though you cant really see how far you have overeaten) thats how I got the character to gain weight. In order to be healthy you need to walk atleast once I day or your health will decrease at one point I started using the gym once a day to make sure I was healthy as I was nearly 500kg. With the friend situation don't take her on walks and over feed her once a day atleast she should be gaining then. This is what I have learned from playing the game.