This is a common situation that comes up on relationship forums.
Situation: Person A got together with Person B and their sizes in the beginning were smallish-average. Some time goes by and Person B is now 50-100 lbs heavier. Now Person A is no longer physically attracted to Person B. Sex is completely off of the table. The question inevitably becomes, "I'm no longer attracted to Person B. What do I do now?"
First of all, take a deep look at yourself and see why is it you are no longer attracted to Person B. Is it really just the weight itself or are you no longer attracted to someone who stopped caring about themselves? There's nothing wrong with either answer. You just need to figure out what it is so you can adjust your life as needed. If it's a personal preference and there really isn't that much love, then you can decide to leave. If it's the fact that you are attracted to someone who takes care of themselves and there is a lot of love in the partnership, then you can brace yourself to stick it out and help the other person.
Next, see if the weight gain is medical. Talk to Person B to see what's going on. With their permission, have Person B go to the doctor to see if it's a medical issue. Then stop by the nutritionist. Stop by a personal trainer to get exercise plans for those that are medically impaired.
If the issue is not medical but lifestyle, look up ways to change your lifestyle habits TOGETHER. Person B is going to need support. Ask them what kind of support they need/want. Some people need a kick in the butt, others need gentle hand holding.
All in all, Person B needs to figure out what's going on MENTALLY. If they used to take care of themselves, what happened to make it stop or slow down? What thoughts are they telling themselves? If they are highly stressed and keep putting themselves down, then of COURSE they're going to gain weight. Get Person B to a coach or a therapist to help through those mental issues. They can help set up a strategy to get Person B back to their normal happy selves. Most likely the weight gain was unintentional. The weight gain was a side effect of something deeper than just putting down the fork. The need is to get Person B back to living life INTENTIONALLY instead of unintentionally.
Word of caution: Person B has to WANT to change. You cannot change anyone but yourself. You can help another person. You can hold their hand. You can lend a shoulder to cry on or an ear to listen. You CANNOT change someone who doesn't want to change.
If you are Person A, all you can do is look at yourself. Why are you asking this question? Then talk to Person B and ask how you can help THEM. If you believe that you cannot help them, then walk away. You aren't doing yourself or the other person any favors by sticking around.