You also need to be the owner of her car. See below from a French lawyer and posted on (the dubious oracle that is) Pistonheads. The question is, how does one prove to the gendarmes that someone else owns a car?
d) Vehicle confiscation
Legal principles
According to the article L325-1-2 of the French highway code, vehicles
controlled to exceed 50km/h or more of the maximum authorized speed,
can be immobilized and immediately impounded by the police.
The immobilization must be pronounced only if the offender owns the car.
The prefect informs the Public Prosecutor who has 7 days to confirm this
immobilization.
Otherwise, its owner can recover it.
If the court pronounces the sentence of immobilization of the vehicle, it is
returned to the owner at the end of the period of immobilization fixed by
the court against payment of the costs of removal and detention in
impoundment.
However, in certain cases, the personal situation of the offender can be
take into account to lift the immobilization, for instance, if the confiscation
is likely to affect the living condition of the relatives of the offender.
Nevertheless, in regard to this last item, each jurisdiction will decide how
to proceed.
Exception
However, according to the article L325-1-2 of the French highway code, if
the offender is not the owner of the vehicle, the immobilization or the
impoundment, must be lifted as soon as a person proposed by the driver or
the real owner can recover the vehicle.
So, vehicle may be confiscated or immobilized after an offense only if the
offender is the owner of the vehicle.
Accordingly, hire agreement with an option to purchase and lease
agreement cannot be subject to an immobilization.