Reduce flame maximum on cooktop

The short answer is no, there's no practical way to reduce the heat output of your cooktop without so much modification you may as well buy a new really horrible one with pathetically small burners. Then you'll hate it because it takes a calendar year to boil water for pasta.

I feel your pain with this, I've had family members break my good kitchen knives and burn the non-stick off brand new pans before I've had a chance to use them, and it is frustrating. Education and a bit of zen are what is required.

You need new pots and pans.

How wonderful to have someone cook for you! It is fun to have a powerful stovetop. She needs better equipment to cook the way she likes to cook. Not crappy pans with melted handles.

Go to a place that sells new pots and pans and get some all metal ones with metal handles. There are lots of nice ones. Buy a set. Unwrap them and wash them and leave them out on the stove with a flower on one.

You can tell her that you wanted some things she could use that would not fall apart when she used them! Because you like to eat what she makes. Do not mention the melted handles. I am sure you already did but it is not too late to not mention them again.

The handles will get hot. She knows that already.

Tip: even though you got new ones, don't throw out the old ones right away. There may be some in there she likes just like I like my old carrot grater and that weird little orange glass saucepan.