Catching her heel would cause her to go face first down the stairs. Wow, this lie is getting worse every time she tells it.
If she fell backwards why would her toe be injured? Her toe world be pointed UP the stairs. Did fall up?
Obviously the tripped on stairs explanation is bogus. So what is the real story? The real part is that something is wrong with her toe. First thing that comes to mind is Gout attack. Which she doesn’t want publicized since it might cause some questions about other medical conditions.