What type of 'fat' are we talking about? Obese or chunky/chubby? Well I know what you shouldn't do, break-up the friendship. That would just show you weren't a true friend and you were just friends with that person for superficial reasons. If your friend is just slightly overweight then you should not care. He or she will realize the weight gain and will do something about it. At the end of the day , I don't understand how their weight gain is affecting you though. It's affecting their body, not yours.
Now if your friend is going through something and they are eating non-stop to comfort them and it's affecting their health. You should say something, but be cautious on how you do it. Do not tell them they are fat. Express your concerns for their health and suggest some fun things you can do together to burn the calories. Don't come off shallow about it
But like I said of it's not a health concern, then you should't care. People gain a little weight, that's just life.
