Here are a few different ideas for you. You decide which one fits your situation best.
Tell her the plastic sink is no longer usable. Even though it is not melted thru, the melted area is too brittle now. And it is likely that adding more hot water will "break thru" the damaged area. You don't want to have her to be cleaning up the water in the cabinets. So from now on she will need to take the dishes down to the river to wash them. A month or so of that and she'll not be making that mistake again.
Of course she may insist that you take her out for restaurant meals...Breakfast...lunch...and dinner. To which, just make sure she pays the tab, with tip. A month or so of that and she'll not be making that mistake again.
Just be ready when you make a mistake. She may find a way to get even.
Or you could replace the sink with new Stainless Steel job. And tell her you are so happy that the old plastic sink is destroyed, so you can finally have an excuse to replace it with a good new sink. If you do this she'll like you much better. And when she makes the same mistake again, it won't cause any damage. Nor will it when you make that same mistake.