Parking sensors on a 500?!
Rear window is terrible and the boot slopes down so the car doesn't just finish at the window.
Last thing I want her to do is to back it into the ramp/step to the front door (built for disabled access when they built all the houses).
The £250 the parking sensors cost are a saving really as the first time you bump it accidentally its going to cost you £250 to have sprayed/corrected.
Plus you have them all the time then and they are helpful, especially for a girl, and she plans to keep the car for 5+ years. They are a bit conservative on gaps, but given the choice to have them or not have them I would pick them.
Being a man and white van driver I just touch park while going sideways following a precise handbrake turn into my space