lol woman are so sensitive when it comes to weight. Obviously if she's trying to gain weight and succeeds... she will gain weight. She should be happy that you noticed, not get mad lol. When I went from a 28 to a 32, I was a very happy man. Gaining weight is not as easy as people think and can be an accomplishment on it's own. The weight loss industry wants people to always think they're too fat.
I say buy her a Double Baconator with super size fries as a "sorry".