in a way, she is correct, fossils are rock formations, though I doubt she was meaning quite that when she said what she did.
Fossils are formed in a number of different ways, but most are formed when a plant or animal dies in a watery environment and is buried in mud and silt. Soft tissues quickly decompose leaving the hard bones or shells behind. Over time sediment builds over the top and hardens into rock. As the encased bones decay, minerals seep in replacing the organic material cell by cell in a process called "petrification." Alternatively the bones may completely decay leaving a cast of the organism. The void left behind may then fill with minerals making a stone replica of the organism. either way, the fossil is a rock, and the material is not in any way organic.