As someone who has been approached by her significant other about this in a negative way and is still pissed about it/still has image issues now because of it...let me add my few cents. It is IMPERATIVE (as mentioned above) that when/if you do bring this up, you make sure and make the main focus of it,not their weight gain itself, but the fact that you love them and care about them and are concerned for their well being.
Don't approach accusingly and don't act frustrated or angry at them about it. If you are angry about it, wait for a time you have better control over your emotions. Chances are this person already knows something is wrong but needs the proper kind of support from people who care. They don't need to feel looked down on and disgusting, they need to be encouraged to love themselves.
Or for a less direct approach, one could start actively trying to incorporate more "active" plans into their routine,and include their partner. If you live in a nice area for it, plan some hiking adventures.
Make it something fun and not "YOU HAVE TO DO THIS BECAUSE YOU ARE TOO FAT AND IT NEEDS TO CHANGE NOW!" I realize that this was a hypothetical question on your part @
Odyne , I mean "you" generally here.
I don't believe it is appropriate in a loving caring relationship to NOT say anything at all when someone you love is at risk of endangering their health. If you are indifferent that is not caring or loving. But I think things like this really need to be handled delicately. It can completely crush a person to hear something like this from their S.O. if it isn't brought up the right way and can cause a LOT of resentment.