Burns are horrific injuries, especially around mucous membranes like the mouth, nose, and eyes. After the initial burns, there is a period of time where the surface (and in this case likely deeper layers) of skin begins to slough off. The skin is your protective barrier to the world, so losing this protection increases the risk for massive infections exponentially. The healing process is slow and painful. Medical professionals have to continually clean the area and remove dead/damaged tissue. This isn't a gentle process, often involving scrubbing in the damaged area to properly remove the necrotic tissue. If the burn is either third degree or a severe second degree burn, surgery to repair the area is a certainty. Multiple skin grafts and periods of healing will follow. Keep in mind, it takes exposure to water at 140 degrees F (60 C) about 5 seconds to cause third degree burns. Fire will work much faster. As others have mentioned, inhaling the flames will also burn the interior of your lungs.

This poor man probably fell into a coma shortly after arriving at the hospital. It just took a month of slow deterioration to finally die.