Alex: I wouldn't rule out a "glitch"
causing the problem. If her wall switch is
on the same circuit (or possibly the same
phase) as what ever is causing the glitch
and the others aren't, the glitch might not
be strong enough to cause the light to turn
on.
It also might be a defective switch, or a
phantom on command. Why don't you first
try changing the address for her switch?
If that doesn't solve the problem, then try
swapping her switch with one that works
properly and see if the problem stays in
her room, or moves to the new room.