This is actually someone doing a boil-out.

The oil is emptied and the fryer is filled with water and a cleaning solution. Then it’s boiled for a while until it’s clean.

The problem is that it’s prone to boiling over and making a big mess everywhere else, as well as possibly contaminating any nearby surfaces with chemicals.

Put a couple of baskets full of ice over the fryer and the steam melts the ice, dripping cold water in and preventing it from boiling over.