I'm going to be the meanie here because someone has to say it.
Your wife lacks the physical and mental coordination to ride and should stop either permanently or until she receives further training. If her reflexes are bad at 0-3 MPH when something unexpected happens what makes you think they will be better at 60 MPH?
Let me not then die ingloriously and without a struggle, but let me first do some great thing that shall be told among men hereafter. ~Homer