Cylindrical lithium ion batteries can catch fire if they are punctured, left under the sun to bake for a day or shorted. I'm sure there's a whole lot more ways on the list but the most common failures listed above are from user stupidity or a faulty overcharge prevention circuit which are almost non-existent. Nowadays, even the cheapest chinese cells have built in pressure relief valves so that even in the worst case, they will "only" catch fire.
I'm not one to play online detective but why would she put her laptop on the floor two feet away from her front door? It looks more like she kicked it there after it started burning and if she really has a reason to hide that, who knows what she really did to it.