This is a tricky question to answer as it is easy to hurt her feelings and I have no idea why she has been picking up weight or if she has been overweight for a long time, etc.
Very often there is a direct connection between weight gain and emotional problems. If I were you I would try to understand the reasons behind her picking up weight. Bad eating habits and lack of excercise do play a role but try to understand what is behind it. Maybe she feels unappreciated and simply stopped caring what she looks like thinking it makes no difference. Maybe she is turning to (comfort) food to make her feel better about something that is bothering her.
In other words, her weight gain is more than likely only a symptom of an underlying cause. If you try to address the symptom in isolation it may backfire. Try to isolate the cause and once you know what it is you will know what to do.
Wishing you all the best!