Girlfriend gaining weight?

I'm worried about my girlfriend. When we started dating 2 years ago she already was a bit overweight (~5'2" and ~140-145 lbs). But since than she has gradually put on another 15-20 lbs because of fast food and snacking. She is also very lazy to exercise. Sometimes she comes with me to the gym but she basically just walks on tredmill and then goes to sauna. I'm worried that having so thick layer of fat around her belly isn't really healthy. What can I do?

Girlfriend gaining weight?
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