25 pounds in a few months is a very significant gain. Is there something going on with her medically? Has she had any life-changing events occcur in recent months? I think primarily, I would be concerned about her physical/mental health.

Assuming there is nothing wrong with her that normal people might look upon with compassion, I think it is NOT unreasonable to be alarmed by such a large weight gain in such a short amount of time. Men, unlike women, can't really fake sexual attraction, can they? I know it will be difficult, but you will have to tell her what the problem really is. There is no way to say it that will make her feel good about it, but do try to be as kind as you can.

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