@Gemma290 I can see why you would want to get the warranty to work on your behalf, but you are running a business. If it were me, and this has actually happened to me, I borrowed a neighbor's truck, and purchased a stove the same day that our guest notified me the one in my unit wasn't working. Then I brought the stove to my rental that same evening and our guests had a new stove. I still refunded her one of the nights for the inconvenience.
If you don't want to buy brand new for a rental unit, then see if there are higher end ones on facebook marketplace, but guests need working appliances.
With regards to having to offer a refund: our log home is in the mountains. When we loose power, even for just a night (due to heavy snow and power outages) we offer our guests refunds for that night. They still have a working fireplace, but a lot of southern's want that gas furnace on top of the fireplace, and I don't want to do anything that might result in negative reviews. Every once in a blue moon you are going to get a negative review no matter how lovely your rental is, but at least in this case you are going to want to bend over backwards since she doesn't have a working appliance for more than a day.
I don't mean to sound unsympathetic, but the fact that your guest is using the oven to cook things she can sell isn't really your concern. She is an entrepreneur, as are we, and as long as she takes good care of your rental, how she uses the stove is not relevant. You are blessed to have a long renter.