Tricky question: theoretically, if she gives him the keys and asks him to pop out for a loaf of bread whilst he is under the influence of the medicine she may be guilty of aiding and abetting the offence:
'Whosoever shall aid, abet, counsel, or procure the commission of any indictable offence, whether the same be an offence at common law or by virtue of any Act passed or to be passed, shall be liable to be tried, indicted, and punished as a principal offender'
The Accessories and Abettors Act 1861.
Whether or not there are specific clauses in the Motability agreement which cover this I know not.