If you did buy something at her bake sale in Hotland, then the fight won’t occur (of course, you’ll also be out 9999G, so most people don’t buy anything there). She fights you for two reasons– one, someone offered her money to fight you, and two, because you didn’t buy anything at her bake sale in Hotland. This item comes in handy during the battle with Muffet. This healing item can be used during any battle or when in the overworld, and is not one of a kind so you can buy as many as you can afford and/or carry. The Spider Donut can be bought in the Ruins for 7G or in Hotland for 9999G.