Gaining

becomingoverweight:
What causes the health issues associated with obesity is excess visceral fat. Having too much of it would probably give someone metabolic syndrome(diabetes, heart disease, hypertension, etc.). Subcutaneous fat on the other hand is rather harmless even in large excesses. So if someone was to gain only subcutaneous fat, their health would not deteriorate.

Here's a post I made containing links to scientific studies on subcutaneous and visceral fat and how to gain them: Compendium of Fat Studies

I've come to the conclusion after reading a lot of scientific studies and reports of gainers drinking heavy cream that dairy(excluding ice cream due to high sugar content) is the healthiest way to gain. It would appear that it only makes you gain subcutaneous fat and can even reduce visceral fat. Many gainers use heavy cream to gain. Lower fat creams as well as cheeses work too.

Another healthy way to gain would probably be with shea butter due to it's high amount of stearic acid(has a healthy effect on fat distribution).

It's good to be worried about her health. It's rather responsible with this fetish. I don't think you're being too cautious.