Let her hang out for a few years with the fat chick and she will be a fat chick too. Most skinny chicks hang in a group of skinny chicks, with one or two fatter chicks. You get one on one and the skinn bitch is getting fat.
Look at it this way, you got the skinny chick in the office with 5 five fat chicks. The five fat chicks go out to lunch every day, the skinny chick goes along to fit in. At first she eats salads. This leads to chicken, that leads to beef. Next think you know shes dropping two big macs, or chinese food every day. Fast forward two years the new skinny chick gets hired, now working with 6 fat chicks.
Point is, if you dont want to be fat, dont hang out with fat people.