On the eve of the celebrity-studded FireAid benefit, 19-year-old college sophomore Lucy Heffernan visits what was once her Altadena neighborhood—devastated by the L.A. wildfires.

“I heard a pop go off. By the time I came out of my room, the whole room was already on fire. It was like the flames were too big to put out. We just had to get everyone out of the hous ...