Trunk racks are a pain when you want to get into the trunk. The bike may obscure the rear lights. If she is tall enough, just get a roof rack and leave it on so she never has to deal with it. If you get a good rack (I like the Thule Sidearm), it is very simple to get the bike on and off. Also, the better roof racks have locks, so she does not have to worry about leaving the bike on the car for short periods of time.
If she is not tall enough, have her keep a milk crate in the trunk to stand on.