projectpolemics: Also, muscle is way denser than fat, so if she's chubby and active she'll bulk up quite a bit.
Hit the nail on the head right here, if she's doing more strength training then even if she does burn more calories than she takes in, she'll still bulk up. Cardio and Aerobic exercises are what really burn fat, so if that's what she's focusing on then it'd be tough for her to continue gaining without really increasing how much she eats.
My ex did this. She wanted to lose weight and build muscle, so she started lifting weights. I started telling her she needs to eat more to build muscle. She did. And she blimped fast. 50lbs in about 6 months, while trying to lose weight. Use it to your advantage!