OP, you need to keep her in your life if you can.
Act like you still want to be friends, and get really chummy with her. Try to do this in a non pathetic way, it's possible, but it's a case by case basis so there is really no advice for it.
And then what you need to do is bake. Bake a fuck ton. Bake so well you become primarch for the baking marines, with Paula Dean as one of your top commanders. And once you've established yourself as a good baker, bake for her. But you need to do the lazy thing in baking and just pack everything with butter and sugar for it to taste good. This is where it gets hard, because too much will make it taste like shit and will give away your plans. But not enough, and the plan won't come into motion fast enough. Though since you are now lord Bakerus Imperialus, you should know exactly how to make things taste delicious whilst being 5 times the calories they normally would.
Finally, you need to start giving all of your delicious baked treats to this dumb cheating cunt. Just fucking drop something off for breakfast and dessert. Croissants and pies are a wonderful tool for revenge. She'll start to gain wait over the weeks you do this, getting fatter and more disgusting as time goes on. If you do it right, it'll be fast enough to encourage tons of stretch marks. She may blame your baking on this, but you need to have some bullshit healthy recipes and faux trendy diet facts to support why you're actually helping her. And then this is when you can start to fill everything with as much calories as you possibly can, since she will trust your baking.
Very soon she will become engorged, and so fat, not even fat fetishists will want to touch her, less they be consumed the gravitational pull she has on her surroundings.
Even if she looses weight, she'll had been fat enough to look like a deflated balloon, and thus still disgusting.
No one will love her, no one will want her. She will die alone.
Then you will have won.