Children Under 3 years old
In the Front Seat
The child MUST use the correct child restraint.
It is illegal to carry a child in a rear-facing child seat in the front if there is an active front passenger airbag. In this case the airbag must be deactivated or the rear-facing child seat must be placed in the rear.
In the Rear Seat
The child MUST use the correct child restraint.
It is the driver's legal responsibility to ensure that the child is correctly restrained within the law. Children under 3 years old cannot be carried in a car that does not have seat belts fitted.
The only exception is that a child under 3 years may travel unrestrained in the rear of a licensed taxi or licensed private hire car that has a fixed partition separating the front and rear seats, if a child restraint or seat belt is not available. However this only applies on an unexpected journey.
Children aged 3 to 11 years and under 135cm tall
In the Front Seat
The child MUST use the correct child restraint.
In the Rear Seat
The child MUST use the correct restraint, where seat belts are fitted.
There are three exceptions where a child may travel in a seat belt rather than a child seat, if a child seat is not available. These are if the child is travelling:
in the rear seats of a licensed taxi or private hire vehicle
if the child is travelling a short distance and the journey is necessary and unexpected
if there are two occupied child restraints in the rear prevent a third one being fitted.
In addition, children aged 3 years or over may travel unrestrained in the rear seat of a car that does not have seat belts fitted, provided it was originally manufactured without seat belts.
It is the driver's legal responsibility to ensure that the child is correctly restrained.
You need to pass this information on. You would be to blame legally if there was an accident and the little girl went through the window (which she would.) If you are agitated by this (as anyone would be) then your driving will suffer, making it even more likely there'd be an accident.
I wouldn't give her a lift any more but I would tell her why, in a text or email.