...I think, to really hurt her, you could tell her that you got it and you use it all the time while shagging your boyfriend... whom you were seeing at the same time you were seeing her...?
And: that's a blatant lie, if she does something like that, than she's definitely NOT over you, but has a nostalgic moment of the kind "if only all went differently" or such.
Show no reaction whatsoever and let her disappear from your life.