Yeap, disable the car or as if you say it's knackered anyway, bung a potato up the exhaust or stick a bag of sugar in the fuel tank.
She'll take it out, break down, get all upset and flustered, and hopefully the experience will bring it home.
We had an elderly neighbour of 91 who wrote my wife's car off reversing out of her drive, across the road, stoved the side and rear quarter in, then drove off leaving half he bumper behind.
A neighbout up the road (a gp) saw it happen and when she returned it was agreed we'd talk to her together - she denied all knowledge of the crash, even whne confronted with her own car's damage.
Most taxi companies will allow you to set up an account with them and all the m-in-law will have to do is phone them, go to wherever, then sign the docket for the journey. At the end of the month she pays the bill, no messing about.