Done this before?
I have thought up nearly everything before.
what kinda car does he drive?
grab a cosmo/elle and rip out a "perfume strip" and plant it in his car...it will just be a piece of paper(small, just a piece, not whole page)
whenever she gets into the car...they will fight..."who the frick were you driving".....his lack of answer will only lead her to believe that he's hiding something.
if you could grab a condom wrapper and plant it, when she starts looking and sees the wrapper....he dead