Please don't risk anyone's health this way. Marda's excellent post has great information you should heed unless your husband wants to end up in the ER. You do risk offending your guest but frankly I would pitch this food when she isn't looking and ensure anything else she cooks goes into the fridge within two hours -- at the longest!
My daughter just did a huge science program on foodborne illnesses and food safety. Two hours is the limit; after that time, even cooked food begins to develop potentially harmful pathogens. Two hours, not 12! The person who posted that her family uses the "if it looks good and smells OK and tastes fine" rule is asking for someone to get seriously ill; foodborne pathogens do NOT usually taste odd, smell funny or produce anything that looks strange. Her family, like your relative's family, is likely somewhat used to eating food that is going or has gone "off," but they still are risking illness.
Never leave things out "until they are completely cool." Two hours to cool at the absolute most, and then into the fridge. It is a myth now that "hot food makes the fridge work too hard." Any modern fridge can handle it.
Check the Centers for Disease Control website (www.cdc.gov) and see how many people each year are sickened or die from foodborne illnesses. Many of those illnesses of course are caused by foods that are contaminated in the field or in preparation, but poor food handling practices are part of the mix too.
One last thing: We have relatives who used to leave food out. All of it. They did not even own a working fridge (and they were not older relatives -- they were our age and at that time in their 20s!). They figured that since they shopped every day and cooked that night, the food would be OK stayiing out for 24 hours as leftovers. And then they began to wonder why onef them had consistent, painful digestive problems. Yes: He was eating bacteria-laden food every single day. .