The problem is not the weight. it's the behavioural change. Look I've always been bigger, but by bigger I mean 20 pounds, that I gain and lose over and over for 25 years, not 80. And I can tell you that extra 20 goes along with a depressed attitude, no fun and not feeling great about things. And that's just 20 pounds. No one gains 80 pounds without a psychological issue.
Ignore the weight. Get counseling. Clear out the cupboards. Spend time together. See what happens. 
Then again maybe she senses your lack of attraction and its a vicious circle. If that's the case then maybe counseling won't be enough. But when you leave her you gotta know she is going to lose that 80 pounds, glow with new found love and happiness and look smokin' hot again for some one else, so be prepared for that.