((((((((((((((phantasmagoric)))))))))))))))
It's a hard situation, I'm sure, but it sounds like you are wanting to handle it in a wonderful way. Definitely don't want to trigger eating disorders and/or more SI behaviors. I think what everyone said about together is a really good way.

I would imagine that she might already be concerned that you might find her less attractive due to her weight gain. Perhaps you can express to her that you do find her attractive as is (I have a feeling that you are already doing that) .

Also, it doesn't have to be all about diet and exercise. It can be learning how to cook this dish and that dish, learning how to cook healthy and wanting to get out and enjoy the fresh air more. Take time to enjoy things together. Keep us posted.