How do you deal with it?

[bluetech]

Look, being fat does not cause illness and death. It increases the risk of it. This is a subtle but very important distinction. Since it is a matter of risk, it is simply an issue of mitigating that risk.

Look at it this way. Imagine you were dating a skydiver. That person's lifestyle choices put them at significantly higher risk of dying than somebody else. Every time they jump out of an airplane they risk it all, but there are steps they take to mitigate that risk. They get great satisfaction from doing what they do, and making them stop would be taking away an important part of their life.

Now you, as this person's significant other, are understandably concerned for their health and well being. But if you simply demanded that they stop doing this thing that is enjoyable and deeply satisfying, nobody ends up happy. But it is possible to be supportive while showing that you love and care for them by  helping and encouraging them to take the steps to mitigate the risks as much as possible.

The skydiving analogy can be taken one step further. You have to make sure that the person doing the risky behavior is doing so because they enjoy it first, and you are simply sharing in that enjoyment. The creeper FA who secretly sabotages their girlfriend's diet even though she explicitly says she doesn't want to gain weight is like somebody just pushing another out of an airplane because they think skydivers are hot. Not a good idea at all.