I keep a box of baking soda under the grill in case of grease fires. It's saved me a few times, usually when cutting corners and grilling the pre-made burgers.

Generally I try to clean my grill once a month in the summer season, but admit I neglect it in the winter months. Last year we had a surprise 40 degree day so I jumped on the grill, without checking it for buildup. I didn't pick up on anything when I threw the burgers on, but 5 minutes later it was full flame.

Threw the baking soda on it, shut off the gas, and pan fried the remaining burgers... have the grill a good cleaning the next day and no issues since.

At least your FIL didn't ask for the beef well done like my did.